Try and buy the best you can afford when it comes to suspension, I cut courners on my suspension and bought Pi rides not great and at some point I change so Spax would be good to go for.
Just springs doenst limit how low you can go just Id say if your going down just on springs stick to 30mm-40mm any lower and lower and a full kit with sports shocks would be idea but no the end of the world if you dont.
Look to having a lower OMP strut brace to tighten the front up their under £30 from
www.rallydesign.co.uk and everybody that has one would probally recommend them.
Poly bushes would be another good upgrade over aging factory or pattern parts but again nice to have but to end of the world if you dont have them.
Worth noting that once you have had the car lowered it may take a while to settle down to its final ture ride height, when I had mine done I had to go back to the garage two days later and the fitter siad Wow thats settled down abit hasnt it. So dont be upset if it doesnt look spot on the day you have it done.