I have purchased many parts (all bearings, seals and a few other things) from the second seller you listed and was more than happy with their service and the quality of the parts.
Imho Allballs is a good brand, and all their products i've purchased have been of good quality so either way I feel you would be satisfied.
If you don't feel confident with your mechanical ability I wouldn't advise taking apart the shock to change the seals. As I recall you may need to grind the head off the shaft to remove the nut which holds the seals on, and therefore need to use loctite on re-assembly.
There are many 'how to' articles on the net which are relatively easy to follow if you decide to do it yourself. If you've rebuilt shocks before you'll have no problems.