Well, actually I'd try adjusting the Bowden cable first (while the owner isnt looking...in case you fix it, he may want more $$
If that yielded some improvement I'd probably jump on it. Heck even if it didn't, I'd still probably buy it. The rock hard brakes indicate possible vacuum pump rpoblem, or severe leak, which can also affect the acceleration problem. So, in short I'd go for it. $600 isn't that much for an entire car, and like they said if its garbage, you can dismantle it, and ebay it. So, what are you waitng for...go buy it and post some pics!
Wait!...Did you say it has no Benz emblem??? Forget it, never mind, look elsewhere. j/k