My ST is currently running more or less exactly what you are after.
The previous owner had a Stage 2 Code Red on it only and it produced 269.5bhp on that. Since then I've added a cat-back Mongoose, Airtec Gen 3 Intercooler and CAIS and probably running now around 280ish. CAIS, cooler and exhaust set me back in total £840 plus fitting (mate's garage) and the map was probably about £400. So just over £1k would get you a solid stage 2.
IMO, you definitely need an intercooler if having a remap done, more for safety than anything else. There is no real need to have a custom map written for it although it depends where you are and who is writing it for you as I think MSD can write them for it or adapt an Evolution map if necessary. Get onto focusstoc, loads of info on there and the membership is worth it for the group buys alone I saved about £100 on retail prices combined for the intercooler and exhaust!