Dave, are you looking for a vintage ride, or back road blaster that sticks to the lightweight ethos of the original Seven? If you want vintage, then look at an early Caterham, Pre-Lit Westfield, or Lotus. Big budget spread between the three, but they'll give you that experience. Move to more modern variants and the things do begin to change. Engines go from 2 valve to 4 valve, carbs and points to programmable engine management systems, options of roomier cockpits, IRS, alloy 4-pot calipers, and leather or carbon fiber seats. You can't go wrong with either style, but the driving experience is a little different.
As for replicas; if you are a purist, buy a Lotus. Semi-purist, Caterham or Birkin. Just want something fun that follows the Seven ethos, then things open up. Most of these cars aren't replicas per se, but rather interpretations of Chapman's concept. Some stray from that a little further than others, but they all have merit. The thing is to find the one that best meets your criteria.
-John