Here we go again
... (I'll be nice just because we haven't had an ASR post in a little while), but in the future, search the forum, (use the Google domain advanced search function since the PB search function is just terrible)
1. "First off, how is this durability-wise? I don't want to fire 500 rounds only to have a completely broken trigger box soon after."
As for the durability I haven't really heard any stories about lemons. The only problems that I have had in mine in terms of durability have been:
a. The stock broke after a few drops/impacts to the butt, the latch stopped working.
b. The stock hop up died after a little while, a lot of barrel changes and a few buckings on a PSS210 spring; not unexpected on a clone gun.
2."Along with that, how is the stock performance? From the research I've done, a lot of mixed feelings have been shared."
There are too many variables in the manufacture process for them all to be truly standard out of the box. but mine was around 425 when I got it out of the box and after a short list of home brew DIY mods and some fine tuning with about $5 worth of materials I had it around 485-500.
3."Also, are upgrades hard to do for this gun? I've heard that some tinkering is required in order to find the best part combination."
ALL guns will take tinkering to find the proper combination of parts. The ASR is all L96 from the barrel spacer forward (with the exception of the magazines, they are proprietary as far as I know). The gun is all APS2 fron the receiver down. APS2 springs, trigger box, piston, piston head, spring guide etc.
On your budget I would get the gun (~110), cheap scope (~30), so that leaves you about $60 for the parts. Personally I would get a Madbull v2 barrel (~30), and a piston/piston head (~30). Anything else you want to do can be done for the change you can find in your sofa and a few searches on the forum (I know I posted the DIY that I did in one of my first posts on the forum).
3. "And if you have any other recommendations that you feel are better than this, feel free to leave them."
A springer is a springer is a springer, find the very few gun specific attributes that you want in a gun and buy based on that. Operation of spring guns will be the same, principle is the same, find the one that fits you best.