This will depend on who is doing the work if it is you or a garage. Some fairly major and expensive garage jobs can be done easily by a competent home mechanic, e.g. clutch change, head gasket, timing belt etc. and parts are not expensive. However if you are paying a garage to work on it even changing the brakes would give you a major repair bill. The chassis and bulkhead condition are the things most likely to give you cause for an expensive bill. But if you weld even that is not too bad depending on where and how much of the chassis in need of repair. As long as you carefully check the condition on purchase and budget for the repairs you could do quite well.
Friend of mine bought a 200tdi 110 station wagon that needed a clutch and a rear cross member it cost 3k but it was bought as an MOT failure that didn't drive. Was a bit of a risk as couldn't check the gear boxes but after another £500 for a cross-member, clutch, some service items (fluids, UJ's etc), and paint job, he couldn't have bought an equivalent vehicle for the same money. However, all of the work was carried out by us over a couple of weekends so there was no garage labour charges. If paying someone else to do the work it would not have been worth it and would have cost a lot more.
You mention a td5 in the future but even the newest of these is now be an older vehicle. Any defender that you buy I would expect to need some work and to change minor service items (uj's, drive flanges, suspension bushes etc) so would probably want to budget another £500 in parts just to give it a thorough service once purchased.
Whatever you do check, check and check it over thoroughly, including DVLA MoT records, HPI etc, and if in doubt walk away...there'll be another one
This is the best advice, check everything repeatedly chassis, bulkhead, and ID being the most important. If you do not know what you are looking for in a defender consider taking someone with you who does. Buy the best condition you can afford, ignore the mileage, and expect any of them to need a bit of further work.
