The general guidline for the LR is a pound per gallon, so a 55 would use about 50 to 60 pounds. More doesn't hurt, just takes up more space in the tank. The best LR is fully cured from a muture system. Unfortunately this also means limited availability and $$. LFS rock is also expensive and probably not as good. Your best bet is to try and find someone getting out of the hobby or downsizing. Craigslist and local reef clubs can be a good source for this. The least expensive choice is to buy dead/base rock and use a few pieces of good LR to "seed it out", meaning to put in a few pieces and let the bacteria population slowly grow onto the dead rock which in turn will become live. This takes a while but abslolutely does work.
As for the tank: If you catch it on time the local Petco has a sale a couple of times a year. $1 per gallon of size therefore 55g for 55 bucks. Brand new. regular price is about $100. You could also look on the craislist and reef clubs for a used tank.
You may get negative comments about a 55g as the dimensions aren't ideal and they can't be drilled etc etc. Personally, I use one and am quite happy with it.
Good Luck in your search.