Full boils for 5 gallon batches is in the 6-6.5 gallon range, plus you'll want additional space to avoid boil-overs. I would say an 8 gallon boil kettle (BK) would be a good minimum, but bigger would be good also. You'll need an additional pot to heat your sparge water in, or a separate cooler to put your sparge water in - usually you're old brew pot works for this (plus maybe an additional smaller stock pot). A large turkey fryer will work great as your BK and can frequently be found for cheap in classifieds/craigslist, sometimes you'll even get a propane burner to help with outdoor brewing.
A mash/lauter tun (MLT) setup. Cooler systems are popular and relatively easy to build. Round or square coolers. High temp spigots for your cooler setup (brass, SS, high-temp plastics??). Again, classified/craigslist is a great place to watch for these items. The cooler you pick should be a minimum of 10 gallons (40 quart) if you ever plan on brewing high gravity beers from all grain; bigger is fine but don't go ridiculously huge. It's also okay if it's not in PERFECT shape but you don't want a beater either.
Hose that fits your MLT spigot to drain into BK. You'll need to filter the grain from the wort using something: false bottom, bazooka screen??, braided SS hose.
At some point during your brew day you'll be using your sparge water, MLT, and BK at the same time so they will need to be separate vessels.
Quick minimum costs off the top of my head would be something like this:
BK (classifieds): $35 (HLT, additional $35) -- I'm thinking used turkey fryers
MLT (classifieds): $20 -- I'm thinking used square cooler
Spigot cooler setup: $35 -- I'm thinking a place like bargainfittings for 1/2" SS
Hose: $10 -- I'm thinking 5 feet of silicone tubing 1/2"
MLT filter: $10 -- I'm thinking braided SS hose from Home Depot, but would opt for something more sturdy.