Min tank size I would start with would be a 75....55's are nice, but the 75 will be a little more forgiving and easier to aquascape as the 55's are quite narrow......You might want to look at used tanks, because you can save a good sum of money if you find what your looking for......A big key to the hobby with the expense issue is to research, research and research and then buy.........
An important piece of equipment that would be on the top of my list is an RO/DI unit.....You can have the fancy, best skimmer and lights in the world, but without good water to start you open the door for headaches.....A sump doesn't have to be big, fancy or anything of that nature....If your a DIY or have a friend or 2 that are handy you can save yourself cash again by building items yourself and it's 2 fold if you know what I mean.....You can do a sump out of a simple rubbermaid container you can get at Walmart or go something fancy like PM (Precision Marine) or even nicer MRC sumps and such and there are others out there.....A skimmer is a very key item as well....Regardless reef, FO tank all tanks can benefit from a skimmer and that is one area you shouldn't skimp on.....Again your choices are numerous, but you have to decided on what type of skimmer best suits your skimming needs? Wet skim, dry skim, ease of operation and use, and how much do you want to adjust the skimmer as well??? These things all need to be taken into consideration and don't be fooled if someone tells you this is a better skimmer because you don't have to adjust, and there are never 2 tanks alike, so what works for 1 might not work for you or I.
Lighting is debateable.....I wouldn't buy junk lights or nothing like that, but would again need to know what your goal or plans are for your tank and yes MH's will allow you to keep just about anything, but then comes costs and there are ways around that as well.....