I'd personally just go down on some 30-35mm springs then do uprated dampers at a later date as you said.
Standard dampers are never too bad for lowering..my Saxo i had a couple of years ago was down on 40mm springs (just some cheap Spax ones) and it was still on them when i sold it nearly 2 years later on standard dampers.
Something i keep meaning to ask, i'm pretty sure the Leon has rear springs doesnt it? as well as a beam?
Only reason i ask is because i've only had the car a week or 2 and my Saxo had torsion bars on a rear beam with dampers but no springs, and my MR2 had springs and dampers so just wanna clarify.