The old iCandy prams have the same universal/multi-brand adapters as they do today which will take any current infant carrier type seat by Maxi Cosi, Cybex, Britax, Joie (not i-Juva), Besafe, Nuna or Avionaut.
If you have adapters already you could always take the pram to a baby shop with the adapters and ask if you can try any of the display models on it.
Personally I reckon the basic ones are usually best value - Cybex Aton B2, Britax Babysafe Core. They last long, good safety rating, comfy and easy to use with the pram compatability. The seats are normally less than £100 and you can get a set with base for less than £200. (The Britax is a bit more but their base is a bit less).
Maxi Cosi Cabriofix i-size is often placed in this category but be aware it has a much smaller height limit at 75cm so might not last the full 12m.
If you wanted a bit more of the comfort/ease of use features you could go for Cybex Cloud G (recline, optional rotation base), or Avionaut Cosmo (extra light, great newborn positioning).
For an all singing all dancing one you could look at Doona (not pram compatible, but comes with its own wheels) or Besafe Go Beyond (the base can also be used for a rotating toddler seat up to 125cm, currently the only such seat on the market, although another should be coming out this summer).