Tough call ... it depends very much on your how obsessed you get with having your bike running and looking 'perfect.' Also depends a lot on luck and patience.
If you can find a bike with good bones - mostly original parts - then you are talking about $500 to $800 to get it what I consider 'safe.'
Get a manual and do the 3,000 mile service is the advice you'll hear on here - and it's great advice.
First on my list are always:
1) Front and rear bearings
2) front & rear brakes and seal(s)
3) tires - even if they look good, if they are over 5-6 years old, replace them - you can get a set of shinkos for about $50-60 a tire
4) oil, filter; then a short change after I've run it
5) speaking of a 'short change' drop the oil pan, clean - new gasket
6) new gaskets/o-rings for each of the annoying leaks that inevitably spring up
7) new chain & sprockets (depends - but often)
new air filter
9) check the front shocks - usually need seals and fresh ATF
10) check the wheels for wobble, loose spokes
11) check your steering bearing - often okay, but too loose, or for flat spots
12) sparks plugs
13) check ignition timing - new points are pricey for these
14) Check/adjust valve clearances
15) Clean the ever-loving s**t out of your carbs - lots of good threads on here - cleaning the carbs can not be emphasized enough.
16) After a thorough cleaning - synch those puppies (after all your ignition, valves, etc. have been set)
Aaaand, I'm sure I'm missing a few things. In my experience, as you rehab them, more and more things just pop up - but I definitely nail those items (not necessarily in that order, except the final synch of the carbs always goes last) to make sure the bike itself is safe.
One other piece of advice - after you get those basics done, if the bike hasn't been run for year, then run it before dropping any more money into or wasting time chasing leaks (assuming something isn't WAY off).
I've found a lot of problem on these SOHCs that have sat - such as tiny leaks - will resolve themselves after putting a few hundred miles on the bike.
As far as tools - well, that's one of the awesome things about these old hondas - they really are designed for the garage mechanic. The biggest help I've found is a good carb synchronizer - I got a chrome one with universal attachments for about $110 shipped. Well worth the money.
Other stuff - I dunno - I'm hooked on brake cleaner for cleaning up the parts on these ... if it doesn't have the original tool kit, then you'll need to get a plug socket that thin enough to work (at least for the 550s).
A brass wire wheel is very helpful, as well as sh**load of shop towels. Otherwise, I can't think of a lot of 'specialty' tools I've needed for these ... although that said I've had bikes for a long time so I've got tools all over the place so maybe I'm missing something.