A brand new pair of DCOE 40 Webers will set you back about $1200a pair out of the box, plus linkages, plus manifold, plus filters, plus dyno time.
Second hand, the going rate for a pair of Webers is about $500- $600. You should now get these rebuilt, plus linkages, etc.
Dellortos are a bit cheaper, and can be found for as low as $250 a pair. Rebuild to follow, etc. Note, Dellortos are more expensive to get parts for than Webers. Everything is around twice the price, and you need two/ four of everything if you're going twins.
I don't have a high opinion of Solexs, therefore I don't know what they cost second hand, nor do I care.
Another thing of note, the car WILL require dyno time. You can get a half decent tune off the bench, but you will never get them perfect unless it goes on the dyno. After that, you should NEVER have to touch them again.
It is unlikely for you to get away with putting twins on for under $1000. Budget $1500 if you want to do it properly.
Then find a long tunnel, drive through it and listen to the induction noise. Turn around. Repeat. Several times. :)
I personally prefer Webers (I own three sets), some prefer Dellortos. One or two freaks like Solexs. It's up to you which at the end of the day.