Hi Grant,
What part/s need rebuilt? and what makes you think it needs rebuilt?
If its bottom end and top end its more than you would think, a "proper" gasket set/s, head gasket will set you around £120 or a paddocks special for about £50, plus rings and shells if needed plus the time to do it. A good rear crank seal is £20 alone. If i remember correctly it has set me back about £300/400 pounds to rebuild and that isnt including what I paid for the engine (having rebuilt petrol engines a long time ago I thought it would be about £150 all in.....how wrong was I....

).
If its just top end (cam shaft/head gasket) then its not too much of a problem but if the bottom end needs done then the engine has to come out.
There is a lot of time involved to do it correctly, checking the bores and tolerances of crankshaft/camshaft and if you are an experienced mechanic then you could do it in a day or so but if it were me I would need to take a few days off work.
For a minimal costing you could get a price online from Paddocks/Craddocks for a bottom end gasket set, top end gasket set, a set of standard standard rings (just for cost only mind, you cant tell until they are off and checked ), a set of rings (again cost only) and a set of head bolts. Buying a good quality Elring set (thats what they come badged as direct from Bearmach) will cost you a lot more but good quality (incidently you can get elring sets from Dingbro).
My advice if you are going to do it is either do it right and take your time or buy a good second hand
running lump with known history (for a vehicle in constant use and no alternative it would be my prefererred option).
My best advice is take someone with you at the prospective purchase who really
does know what they are looking at!!!
I hope this helps, I dont want to dampen your enthusiasm just trying to prevent you from either doing something twice or buying a something you might regret.
All the best
Grant