i've been loking into suspension kits for my focus and the koni street kit is very good value for money (under £300 for my car) and is springs and fixed rate dampers with a lifetime warrenty. most other suspension kits i've seen only come with a 2 year warrenty and the really shit stuff 1 year warrenty!!

you can also get the version up from this which is still in your budget but the dampers are adjustible (not in height) so you can have them soft for daily use and stiffen them up for track

hope this helps.