Just purchased a repair kit for the rear calipers I bought from E-bay.

Lots of rubber seals ....

Never attempted anything like this before ....

Any tips, problems to look out for ?

cheers
Jim

Make sure everything is scrupulously clean including your working area.
Some fluid seals have a slight chamfer on the inside which should have the
widest section furthest into the caliper. Use lots of brake fluid or red rubber
grease to aid assembly, do not use mineral grease as it rots the seals.
Be careful not to puncture the dust seal, easily done as they're thin so avoid
using metal pokey things.
The pistons should sqeeze in easily by hand.

Not a difficult job, have fun.

What NigeEss says ^^
Also extracting the pistons really needs a blast of compressed air into the brake pipe connection hole but be careful the piston doesn't fly out. You can try using a foot-pump but that's not so effective.
The inside bore of the caliper needs to be mirror smooth or if lightly scored buy a set of honing stones to dress the bore (Draper do them).
The biggest question however Jim is "Do you feel competent?"
Whilst it's rare for seals to totally and catastrophically fail you are dealing with a safety critical item.
If you haven't undertaken this type of work before maybe you should consider some re-furbed calipers.
OK most of us had to learn at some point but ignorance is bliss and this is a hugely important component.
HTH.
Cheers, Pewe10