Some of the specialist tools are not too expensive, and you might decide that you can justify buying some of them. That is more likely to be the case where you decide to replace a component like for like, and you need the same tool to remove the existing component and to fit its replacement.
For example, you can buy a generic cotterless crank extractor for square taper chainsets from Wiggle for
£4.39, and a Shimano square taper bottom bracket tool for
£11.90. The latter fits Shimano UN55 and UN26 bottom brackets (the most common) and also some other brands' bottom brackets. It can be useful also to have
this tool to use with the Shimano BB tool, especially for removal if you find that your existing bottom bracket is seized in the bottom bracket shell, since it holds the Shimano tool securely against the bottom bracket splines while you apply force.
The other main specialist tools are:
Headset spanners (which you might only need to use once, i.e. to dismantle the headset, check the bearings and races, pack with fresh grease and reassemble),
Cassette/freewheel tools. You'll need to check whether you've got a freehub with cassette or an old style threaded hub with freewheel (explanation of the differences and how to tell them apart
here). I think it's likely that a quality bike of that era like your Orange will have a freehub/cassette. If so you'll need a
cassette lockring tool and a
chain whip. Cassettes have a limited lifespan, which can be prolonged by replacing the chain before it gets too worn, and since they are effectively consumables, it makes sense to get those tools. NB If you find your bike has a threaded freewheel instead, it might be better to buy a new rear wheel with a feehub which takes a cassette (check the rim wall thicknesses - if the brakes have worn them down to 1mm or less, it's time to replace the rim anyway, in which case it would definitely make sense to get a new rear wheel with a freehub).
Cone spanners. Thin spanners for adjusting the hub bearings - you'll need to measure the cones to determine what size(s) are needed. If the wheels spin freely you can probably put this task off for a while, but if they've not been greased for many years, you will need to disassemble, check the bearings and bearing surfaces, and re-grease.
Chain tool. It makes sense to carry a basic multi-tool or separate tools with you on the bike in addition to tyre levers and spare inner tube(s). If you get a multi-tool like the
Topeak Hexus, it also has a chain tool (although the tyre levers on it are not good - you should carry separate tyre levers). Tip - if you have a 3/4" ratchet handle* and a 4mm(?) allen key bit for it, at home you can use that with the chain tool on the Hexus and it's then like having a large workshop chain tool in your hands.
Cable cutters for gear and brake cables and cable housing. The cutters on most pliers can usually be used in an emergency or as a short term fix, but they tend to crush cables and housing rather than cut cleanly.
* Edit - that should read 1/4", not 3/4"
)
