I see what you're getting at
There are loads of singles & a few 2 seaters around but, I don't think anyone here has built a 4 seater anything; I don't see it being too big a deal though, the biggest problem would be transporting it
I don't know of any plans as such but, I'm sure, collectively, we can come up with something...
Regarding price, you
do realise that, by building something from scratch, it'll probably cost close to what you'd pay for a used golf cart; the other thing to consider is prices vary depending on location; what costs $2000 where you are (please fill out your profile so we know where that is...) may cost as little as $500 100 miles away; if you're willing to travel a bit, you may want to try looking a bit farther away, if for no other reason than to cure your curiosity...