Recently (past week) I have been looking into this very thoroughly and I have found 4 viable routes.
1.) The cheap(est) route: Buy a (preferably used) Prius and purchase a PHEV kit from Enginer (http://enginer.us/products/conversion_kit.php). Then, recode the software so the Prius can go 70mph in EV mode (http://www.pluginconversions.com/picc_pressrelease61009.html). PICC also offers a PHEV package but their prices are very ambitious. Between the battery and the reprogramming, this would cost about $7,250 + the cost of the Prius so about $17,500 and would provide a 60mi. EV range.
Pros: Cheap
Cons: It's a Prius, very limited power (and speed) in EV mode, relatively limited EV range, questionable durability of a motor that is being used much harder than it was engineered for.
Although all traditional hybrids could be converted to a PHEV, their EV mode is fairly limited in terms of power and top speed. Of all the hybrids I researched, only the Prius could (be made to) drive over 65mph in EV mode.
2.) The OE PHEV route: Purchase a (preferably used) Chevy Volt. Today, a used Volt an be found for under $25,000 and should (theoretically) give you a 40mi. Range. If you wanted more EV range, you could always add an Enginer kit which would give you an extra 60mi. of range (for a total of 100mi.) for $5,250 (after EITC).
Pros: It's a well engineered system, excellent EV power and speed
Cons: Only seats four, relatively limited range without the extra battery
Altnatives that only do 62mph in EV mode: Honda Accord Plug-in, Toyota Prius PHV
3.) EV only route: From a numbers standpoint, I think the Leaf is the best if you're going this route. After EITC, it is around $23,000 and should go 109mi. on a charge. Apparently Nissan is installing quick chargers in a number of dealerships but I haven't seen any consistent objective information on this.
Pros: It a well engineered system, low center of gravity, cheap, relatively powerful in EV mode
Cons: once the electricity runs out, that's the end of the show; relatively pricey
4.) The practical route:Purchase a Ford C-Max Energi and a 12KWH kit from Enginer. This would give you a total range of 80mi. and a top speed of 85mph in EV mode; total cost, $31,745 after EITC, Ford Customer incentives and all the small print.
Pros: Very practical (can fit a mattress in the back), very good EV range, well equipped
Cons: It's an MPV, relatively meager acceleration in EV mode
Ultimately, you have to take a step back and ask how much range will you (regularly) use and when you need to take the occasional long trip, can you use a different car? Additionally, how much space do you need?
I looked into the Miata EV and they use lead acid batteries which can only endure about 400 full cycles or 700 half cycles. To use a more durable ion lithium battery of similair capacity would add (many) thousands to the cost.
Also, I like the idea of using the C-Max (or any other roomy EV hatchback) as an air conditioned tent (just leave the car "running").