TM MWS has the most aftermarket support. It wouldn't surprise me if you could buy each component individually and build one from the ground up. Pretty much any HK shop will carry MWS parts.
GHK QC has gone down the toilet on their rifles so I would tread carefully. I think the current recommendation is avoid or get a sorted used one. Spares are available from Samoon. Fairly reasonably priced iirc.
I could be wrong but I think the TM MP7 is notoriously difficult for getting hold of spares. That said, it's pretty reliable so not a massive problem until it is. Also, it's not to scale and feels like a toy compared the the WE and VFC offerings.
In summary, yes expect your GBB to "break" but unless you have terrible bad luck it will be what are considered consumable items like nozzles and springs etc which are recommended to have a few spare on hand to swap in should something fail.