Any modern bike can be stripped with a surprisingly small number of tools. If you're going to do a full strip and service you'll need a workstand, as trying to fettle while it's upside down is soul destroying. You don't need to spend the earth on one,
eBay has loads of budget stands, but get one with QR jaws if you can, they make life easier.
Good allen keys. You actually only really need 3,4,5, and 6 mm ones, and the six is only really for pedals and stem bung fixing, the three for bottle cage and rack bosses. Treat yourself to a decent set of long handled "P" type, but double up with short standard 4,5 and 6. There are some bolts which the long reach ones won't get to. The 4 and 5mm ones are the ones you'll use the most.
A decent pair of pliers, bull-nose and needle nose. Just because.
Cable cutters. Invaluable.
Chainwhip and lockring tool. A chainwhip is vital for swapping cassettes, but you won't use it very often. To keep costs down, get a 1/2" drive ratchet with a quick release button. You can get decent ones for less than a tenner on eBay, and you can attach all manner of more specialised tools to it, and the lockring tool is one of them.
Similarly, a Hollowtech BB tool is best in 1/2" drive version. While you're at it splurge a quid or so on a nylon Hollowtech preload cap tool.
A couple of decent cross-head screwdrivers. Get one that fits the limit and B tension screws on your mechs properly to save chewed heads.
A set of seal picks or dental picks(again, a couple of quid on eBay) and a grease gun...
Halfords do one that takes their tubes of grease for not much money.
Have a supply of sundries knocking about... cable saver doughnuts, cable end crimps, outer cable ferrules in 4,5 and 6mm flavour, zip-ties and electrical tape.
A chain breaker is useful, both in your pack and in the workshop. Buy a forged one for home, cast ones break before the chain does. Save yourself the heartache of trying to peen joining pins, get a few quick links and some quick-link pliers. Faster, easier, more secure joins.
Lastly, a tube of coppaslip, a tub of general lithium grease, or marine grease if you ride in sloppy conditions, a can of GT85 (look after the little red straw!) and if you have any carbon components, a tube of carbon assembly paste. Finally, a bottle of 99% Isopropyl Alcohol off eBay is great for any emergency degreasing and cleansing required.
I'd recommend sourcing all of the tools from someone like Superstar Components or On ONe. Their tool range is perfectly adequate for enthusiastic amateurs, and they'll have Bank Holiday deals on at the moment. Wheelies Direct have a sale on tools as well.
