Your choice but I think we are mixing up terms a little. There is base coat/clear coat that requires a clear over the base but you can do a single stage ureathane. Dulux, the original enamel has the top film glossy but cannot be buffed or it will destroy the gloss film. Acrylic enamel can be buffed and is more durable. Urethane is the next step and more durable. It is so tough you can buff after a couple hours of air drying.
I think they could adjust the shade to match the weathered paint but they won’t on a cheap paint job. If the original paint called for clear (coded CC on the paint tag). I would do the clear to match original. I’d also likely do the whole thing but with the urethane. If you have spots that you were just going to clear, you will have a blotchy finish. It will look bad.
Now expect grinding marks, over spray etc. on a cheap paint job. The more work you can do yourself in removing trim and masking, the better. Might turn out ok and might not. Spraying a car is easy. Prep is not. Paint is also expensive so you get what you pay for.
Bottom line is what makes you think you’ll get give more years out of it? In my humble opinion anyway.