how does your score compare with the 80% ranges the schools post (if they post them!)? it sounds like it will be on the lower end, but the 80% range means that plenty of students with those scores were admitted. since you have less w.e., i think the schools would base your admit more on gpa and gmat, so i would definitely recommend it.
as far as prep for quant, just work tons of problems
but be sure to understand why you get wrong the things you get wrong. so make sure the study guide you're using gives explanations. i'd also recommend studying the standard formulas (i.e., area of a triangle, volume of a cylinder, stuff like that) and knowing those cold. you just have to learn to recognize the common tricks--for that you need to study strategy. personally i used a kaplan
book, but many others on here like the manhattan prep. good luck!