If I remember rightly, the cheaper ones (CYMA/Cybergun/Double Bell/JG) have bodies made of ABS plastic which isn't as strong or nice feeling as the nicer polymer ones such as Tokyo Marui, King Arms, (I think) Novritsch and Krytac. The cheaper ones tend to be new for about £100-£150 depending on brand. King Arms (if you can find them) and TM are in a middle ground of pricing about £200-£250. Novritsch also makes a P90 called the SSR90 which is about £300. Krytacs tend to be on the more expensive end I think.
As for mags i've always stuck with King Arms 100 round ones as I bought a box set of them when I got my P90, never had an issue with them feeding in CYMA, Cybergun or a King Arms P90. CYMA make 70 rounders, JG also make 100 round mags. There are higher capacity "MAG" branded ones that are 150 rounders but they tend to be hard to find and expensive when you do. Finally, Novritsch also has 100 round P90 mags.
As @Tackle mentions, you can get a M4 magazine adapter so you can bypass this issue completely. It costs anywhere from £30-£50 depending on the retailer.