I used to airsoft a lot a while back. There are a few things you should consider when buying a GBB.
- Maintenance.
- Availability of replacement or aftermarket parts.
- Cost of use.
- Performance.
- Track record of brand.
If you're looking for something to shoot your friends with, look into some products from KWA, most notably the Glock line. They should all come with metal slides stock and there are tons of aftermarket parts should anything break. They're also pretty cost efficient since they run off of Green Gas. If cost of gas is that important you can also convert them to run off of propane. I bought my Mac-11 used a couple of years back and it still shoots perfectly.
However, if you just want something to fuck around with friends with and not have to worry about maintenance and other stuff, look into a spring gun.