It depends very much on what you want to achieve but an idea that may be worth considering for people who are thinking about an upgrade is to upgrade your existing car.
Let's start by assuming that your existing chassis and bodywork is ok but you just want something a bit more powerful. Why not consider putting in a Ford Duratec (MZR) VVC 2.0 engine with throttle bodies and putting out 195hp or 215hp. This is essentially the same engine as was used in the Plus 4 SS. Or a 220hp version of the Ford Duratec 2.5L VVC engine; fantastic torque for cruising.
You may need to consider wider tyres or improved brakes if the car is a 4/4 or an older Plus 4 but the more recent Plus 4 should be ok.
Other than that some new carpets and a pair of reclining sports seats should not cost the earth.
Then if you were thinking about suspension improvements too, new kingpins and an SSL front spring set plus a rear SSL 5 Link suspension and you would finish up with a very personalized and special car.
All that would remain then is a trip to Italy to have one of Borghi's dashboards installed