Which will be a better choice in terms of cost of ownership/reliability; early 2000 BMW Z3 or Porsche Boxster? I am looking in the 10-15000$$$ range as a second car. Also, how many miles is too many? Any threshold for problems with either model?
the Boxster will probably be more reliable. BMW’s are not known for that, while Porsche has a reputation for being more sturdy than the other exotics.
The Miata is often overlooked…but in the company of the cars you listed it will come up a bit short shall we say. But the fun factor of the Miata shouldnt be ignored. As for your two choices…the Porsche is definitely a sweet ride, I have driven most of them and they never get stale. However the Porsche has an “encapsulated engine” meaning the only part of it that you will be able to see and touch is the filler neck for the oil and maybe the dipstick? They are almost considered “disposable engines” So if yours needs work…lie down before you start getting estimates. The BMW shouldnt have any relaibility issues and the engine is conventional in every sense. Its up to you the Porsche is in a more rarified air when we start talking about repairs… The BMW is conventional albeit still pricey to have serviced. Its entirely up to you I suppose. Both of your choices are expensive to maintain if you dont do the work yourself. Lowest miles you can get is best.
Are you totally against the little Miata? I dont know of any other convertible sporty car that is more fun to flick around. I have driven them all so fire away with more questions if you like.
Google ‘Porsche Boxster problems’ to find out about the early (pre-2000) problems. Later ones seem to be OK. But make sure it has complete repair and maintenance records, Porsches need to be well maintained to not be a money pit. BMWs too. The Porsche is the better sports car, for sure.
And I agree, for the money you can’t beat a Miata.
s2k 
OH YEAH…I entirely forgot about my favorite maker HONDA…the S2000 is fantastic! Dont know about the price range though.
Texas made a great point…those older Boxsters were afflicted with engine cases with high porosity…meaning the engines would literally leak oil THROUGH the engine casing. Porsche was literally giving away free “power units” They replaced the engines on many a Boxster for free…it was well documented so be aware. I still love the Miata… but now that the S2000 is in my mind…that is one sweet convertible too and deserves a good long looksie… How did I forget that vehicle?
Don’t forget about the lowly Corvette. Easy to work on, cheap(er) than an exotic, good mpg on the highway, fun to drive. Usually pampered by someone who never drove it, no rust issues. Cost to own is low, reliable as you can get. Rocketman
Good point about the engine.
At this point in time, all of the early Boxsters (97 though 2000) that had early engine issues have had their engines replaced by the dealer.
I can lay out a list of things to look for when buying a Boxster, but there are entire websites dedicated to doing that for you. Here’s one of the best ones around:
http://sites.google.com/site/mikefocke2/mikesporscheboxsterwebpages
If you can’t learn enough about the Boxster to decide if you do or don’t want one, then all that’s left is a test drive on a really twisty road, back to back with any other sports car you can think of.
Boxster usually wins after that test.
BC.
Of the two choices the BMW is likely to have more problems, but both of these cars will be expensive to repair when something breaks or fails.
Consider the Miata and the Honda S2000. Both are lots of fun; proven reliable and much less costly to maintain and repair than either Boxster or Z3.
There is no such thing as a cheap European car, especially convertibles.
The BMW’s had chronic frame problems where the rear axle (differential) pulled out of the frame leading to a very expensive repair…An RX-7 or 8??
I will second the recommendation to look at a Corvette. Depreciation is really having its way with the newer ones, and they are simple cars that any halfway decent mechanic can work on with a drivetrain similar to what can be found in a similar vintage Chevy truck. It can be tough to find a mechanic who can confidently work on Porsche or BMW because they have done it many times before. This is not a problem with the Corvette with its familiar smallblock Chevy V8 and, in the case of an automatic, its familiar 4L60E tranny. Repair costs on a Corvette are comparatively VERY low compared to the two cars you are looking at, and repairs will likely be infrequent as well compared to the Porsche or BMW. A 10 to 15 year old 'Vette should be in your price range. If heart-pounding acceleration and tons of lateral grip sound like a good time to you, the Corvette is worth a look.
Cars.om on this cartalk message board has tons of used Corvettes . . the $15000 models seem to be mid to late 1990s, many with low miles. Although I’m not really a Chevy guy, the Corvette has come a long way and should be considered, IMO. Most guys I know who own 'vettes baby them, you may get a real cherry. Another thought came to me . . . why not look at a used Viper? Rocketman
Corvettes and Vipers: two extremely fast, athletic, capable vehicles that rarely see their limits pushed by the owners. The Viper also has the same advantage of the Corvette as being a basic, familiar design to a lot of mechanics. I have never worked on a Viper (I have worked on several Corvettes, though) but would not hesitate to do so. A Porsche is alien to me, and a BMW is a rare pain in the butt for me to encounter. The OBD system on domestic cars like the Viper and Corvette are also notoriously easy for any common scantool to access. This can be a problem with some exotic cars. These two would be your cheapest to repair and most reliable exotics to consider, with the Corvette having an edge on the Viper as far as finding, first of all, one for sale, and second, a mechanic who has worked on them.
Check out the Chrysler Crossfire an its twin the Merceds Benz SLK. They are fun to drive and have been around for a long time. Well, the SLK has, but the Crossfire was a rebadged SLK for 4 years. The Crossfire will also be less expensive than the equivalent 280SLK.
I’d avoid the Viper because the initial price will be prohibitive, and if you end up scraping a fender or something breaks that’s specific to the Viper engine, it will probably put you in the poorhouse to get it fixed. There just aren’t enough of them out there for parts to be cheap.
Nice car, but wasn’t the OP’s idea to have a SMALL convertible?