I am considering several different types of vehicles; a Locost, a replica of the VW XL-1 (probably the original tandem...) or perhaps the TREV...
Have you ever built a car before? If not, I'd suggest following a known path for your basic build and only do one thing radically new/different at a time. Seriously, if you're not Chip Foose or Randy Grubb, you're not going to build a VW XL1 replica (heck, even Volkswagen apparently can't build a VW XL1 replica, seeing as they've only built one of each iteration) and if you don't have a college (or Burt Rutan) behind you, you're not going to be bonding aluminum honeycomb to make a TREV chassis.
...there are no groups that I can find that deal, in a meaningful way, with either the TREV or the XL-1
And hence the benefit of the Locost, which has instructions printed out in books and a strong and supportive community of builders.
The same attributes that make a fast Locost fast (light weight, small frontal area, and no power-wasting frills) can make a green Locost green. Mine cruises down the freeway at 100+ mpg, how efficient is efficient enough for you?
On the other hand, you can achieve 100+ mpg relatively easily with a 250cc motorcycle and some serious streamlining, so if building a car is more commitment than you're ready for right now, perhaps you could gain experience with a bike first. If you're considering several different types of vehicles, a motorcycle is one type to consider.