Cheap fox rear
The 8.8 in stock form is a pretty tough rear end. I've seen it handle 500rwhp over and over and over again as-is, with the first thing to go being the trac-lock unit, and that failure mode being just a failure of the friction material. In a few cases, the bearing supports broke. They make rear cover improvements for that. In a few cases, axles broke, but that too isn't terribly common either. Due to the failure mode, however...that one worries me more than the others. It's a c-clip rear end...the axle can find it's way out of the housing if it breaks. In spite of some people worrying about the 4-lug pattern, I've never seen a wheel come off due to lug failure on a 4-lug rear and know of cars that have run into the low 10's on 4 lug rears.
So...all in all, it's pretty stout. If you have money to spend, I'd put a bearing support on it first. Then, the next chunk of cash would be better axles and "everything else" in the center section, as a unit. You might also want to look into a c-clip eliminator kit if the thought of having an axle walk out of the tube scares you as much as it scares me.
As far as control arms...if you want to go budget, I'd just use police and taxi control arms on it and box-weld them with some bar stock. The bushings are stiffer than stock, and the box weld will keep the flex to a minimum.
You havn't mentioned the torque boxes in the rear chassis where the control arms mount. You should be very concerned about this area; and at a minimum seam-weld all that area to strengthen it. I've seen fox cars leave the *** end at the starting line at launch with chunks of torque box hanging off the lower control arms. Not pretty. They make kits for this as well..."battle boxes" and the like. At 500rwhp, and assuming your chassis is in good shape...a weld job should be sufficient.
Springs and shocks and such need to be chosen as well. The stock stuff will work pretty well for starters, but drag springs/shocks and possibly a bag in the right rear will improve matters.
There's all kinds of stuff to buy for the rear if you don't like the police/taxi control arm idea...but you said budget, and for 500rwhp...I really think this is good enough; especially to start.