The fox airs have worked great from everyone I have heard from. I have not used them but a couple friends that go do the deserts in their minis have them and have had no problems.
The only problem is when you just weight them down to much.. like by adding all the weight from an automotive transmission and car motor.. then they don't do so well. Then you just exceed there lift and weight limits basically. So then change to a coil over.
Also when racing they cant hold up so far. Fade etc. the heat gets to them. But for play like I said my buddies little cars kick butt and ride good doing it!
If small enough you could probably do an air up front and a coil in rear.
Keep weight as low as you can where safety will not be a factor but just keep stuff simple as
simple usually saves weight.
For playing and lighter weight cars they are awesome.
No worries about this weekend help your buddy for sure.
Yep know what you want and then get parts.. as things change the more you learn.