They should be able to tell you what spring rates they are going to use (can you not find out from anyone with a similar setup car what they are using as a guide?).
I had a set of gaz golds off them direct a few years back and they did do them at a good price with 2 sets of springs that i specified. Had a couple of issues that i had to return the shocks for and found the aftersales service to be 'ok' but took a while to come back.
Fairly cheap for what they are though so couldn't complain too much, iirc it was a chap called David Lyons that i spoke to regarding technical info as he used to work for leda or something along those lines.