On an N plate car you should be the Mk3.5 (do you have a fuel flap or a fuel cap?)

The seats I don't think will bolt straight in as the 3.5 had some differences to the floor pan.

The bodykit and bumpers should be a pretty straight forward fit, as mentioned above a lot of people bond the kit on to save drilling the bodywork.

The suspension will not go straight on as the hubs are different on the 3.5 than they are on the 3. If you use a Mk3 kit you will also need the Mk3 hubs as well.

HTH