Well my d series was smokin really bad. So I got a DOHC ZC to replace it and it was smokin a little. So I just rebuilt it and I guess I spent about 200 dollars doing it myself. The ZC gets great gas mileage and is fun. It's not crazy fast but made the car a lot more fun. If you wanna spend big money, and a lot of work, you can get a b series, but that up to you. The ZC was pretty easy to swap, and used my old tranny. But you could rebuild your d series, and really enjoy it too! Hope this helps!