the MBX5 in my opinion is very weak in standard form but the pro-spec has all of the Mugen factory hopups included which make the MBX5 a very capapble buggy
my MBX5 used to break nearly everytime out until i bolted some thick alloy strength in there, the front hingepin holder used to snap regularly & the servo saver plates & front shock towers bend!
once you've fitted all the thick suspension plates, shocktowers etc the car will be a little heavier but the prospec includes titanium turnbuckles etc that are suprisingly much lighter than the steel they replace
the pro spec also includes the one piece engine mount that go's some way to reducing chassis flex in the middle of the chassis between the braces as they seem to go a bit bendy in the middle after a while! you'll know when they go bendy as they develop a sudden appetite for clutch bearings due to the inconsistant mesh during the chassis flexing!
i ran one most of last year & liked the car & with al the prospec bits fitted it was very reliable.
you said get a 777, i've got a 777SP1 & to be honest i don't really like it. it has loadsa steering but doesn't feel connected at all to the track, it just seems to get chucked about all over the place, way too twitchy. much preferred the K2 i had as a back up to the Mugen
you should also consider the XRAY XB8 as its quite similar to the Mugen in a lot of the design & you can see this when you look at it but with a C hub front end! better steering than PBS!
the XB8 builds up supremely well & is probably the best kit i've ever built. sorry i can't offer too much in the way of feedback about driving the car as i've only run it a couple of times just to bed it in etc & run in a new Picco P7-R
first impressions are that it turns in well, holds a tight line & rides the bumps fairly well, i'll find out more once its been round Slough a few times
the top 3 are definitely XRAY Kyosho & Mugen (in no particular order
) and any one of these buggys will serve you well, just have a good look over all of them & choose your weapon