I was going to say no to a PS90 due to if nothing else, ammo cost, but the 5.7mm isn't horrible. I was going to suggest some type of .308 but it looks like ammo is about the same price as 5.7mm. Heck if the PS90 really turns you on, get it, but it'd be quite a ways down my list of firearms. I'd probably get some type of old school military rifle, hopefully chambered in .308 or something, or just an M1A.
Why don't they have the regular 50 round mags for the PS90? Unless you live in a scumbum state like NY that is limited to 10 rounds, I didn't think there were restrictions on them for the most part.