First and foremost...pick a rifle style that you like. MP5/AK/M4/etc, etc, etc...
Then narrow down makes and options. CA M15, KA Galil, RS Type56, TM MP5, etc....
Then take a look at the following three aspects:
1. Availability of repair parts (for example...trying to find a replacement M4 grip is easy....trying to find just the front sight of a 249 might be really hard). Same...availability and price of accessories (so models don't have non-hicap mags available, which really turns some players off)
2. Battery size and location. Large SubC sized battery packs rule...way better than mini batteries. LiPo's offer a lot of power in a small size...so that doesn't make it such the issue it used to be. Full stock batteries are nice and easy to get to...a mini battery crammed in a vert grip of a MP5SD is a PITA.
3. How popular is it? An M4 is an M4 and everyone has one...that means everyone has spare parts for them, everyone knows how to work on them, etc... If you have the one and only of something...then you're the subject matter "expert" of it... (not really true but just to make the point)
In general...unless there is no other option...go with the best body/externals/fit&finish you can. Internals can be upgraded, tweaked, tuned. A sloppy receiver or wobbly stock is typically something you can't fix. You'll often find that things either have a great "body" and so-so to shitty internals...or decent internals and so-so externals. The ones that have so-so outsides and insides (i.e. CYMA), and the ones that have great internals and great externals...are priced accordingly.
Gas guns can be fiddly. They may work out of the box...they may run a long, long time without doing much. They may fail spectacularly right away...'cause they get pounded hard. They're temp sensitive (i.e. it's fucking cold right now...don't expect to use it at all outdoors). But they're great when they work.
But don't let anyone talk you into or out of one thing or the other....pick what you want, do your research (LOTS of reading), gut check your budget, find it and have fun.
What's "reasonable" and "good" is entirely up to you.
My $0.02 on your choices...
- so so finish
- well built
- not so compatible with other parts
- good field gun
- minor (short list) things to look at
- good built/fit&finish
- good operation
- big mags that are hard to fit in pouches
- sounds "soft" compared to M4/M14/PDW counterparts...that's a subjective thing I guess
- pretty new...not sure if all the "bugs" have surfaced yet (i.e. the WE SCAR had a bad habit of breaking it's hinge plate with use...that didn't show up until guys really started to use them...other forums are great for seeing what a larger user base is running into)