I've owned my current Westfield since 2003 (my first one came to a sticky and painful end). Despite being used almost exclusively on the track, it's reliable and fairly easy to maintain. As it is very light, consumables such as brakes and tyres last forever despite harsh punishment. Generally speaking, you can spend as much or as little as you want on a Seven - Caterhams, for example, are an extremely expensive way of owning a kit car without any clear advantage over cheaper Westfields. Both are generally well respected and most examples (if constructed properly) offer fantastic performance, even those with smaller engines. I've heard good things about MK Indy and Dax, especially those with bike engines, but I've not driven either myself. The numerous other manufacturers of Sevens are often regarded as being a bit second rate (including Robin Hood I'm afraid).
Mine has the same power to weight ratio as my ///M, but weighs much less than half as much. This translates into a marginally quicker 0-60 time, and far better braking and cornering ability. Flat out it struggles to reach 120mph as aerodynamically it is a brick, but its strengths lie in changes in velocity and direction, not straight line speed. The chap who built mine had a 996 GT3 and he told me that on tighter tracks, the Westfield was a markedly faster car.
If I were to replace my Westfield with another track car, I would buy either a bike-engined Westfield (Hayabusa) or MK Indy (Hayabusa or R1), or a Westfield with a tuned Ford Duratec engine or a 3.5l Rover V8. There can be problems with using a bike engine in a car. Since motorcycles lean through the bends, cetrifugal force mimics gravity, maintaining the line of force parallel with the vertical axis of the bike and hence effectively keeping the engine upright (at least with respect to the forces it experiences which are largely parallel to the vertical axis of the engine). Since cars do not lean into corners (rather they roll away), a bike engine will be subjected to lateral forces it is not designed to experience. The net effect is that oil supply can be an issue. Also, bike engines can lack torque, necessitating high revs to get usable performance which can make driving on the road somewhat frenetic. There is some compensation for this though, as they are generally fairly light.
I would recommend looking at Pistonheads.com. In the classified adverts they have sections for bike-engined cars, kit cars, Westfields and Caterhams.
Whatever you buy, you'll have enormous fun - suffice it to say, I've owned my ///M since 2009 and I still take both my Westfield and my ///M to most trackdays
