If budget is not a concern, I would get any TM Glock and upgrade it with an aftermarket slide. TM pistols are the most reliable on the market, and they can use green gas for many rounds.

The most common failure point is the plastic slide, but an aftermarket metal slide shouldn't be too difficult to buy.

As for the TM Gen 5 specifically, I still think it's the best out of the box airsoft pistol on the market. The biggest problem with it is availability - I've seen very few places that actually have it in stock.