i have a set of 36mm drla dellortos on an 1800 kombi motor they are great carbies, the equivalent in idf webers is 40mm. you will have to pay about $800 for carbies manifolds linkages and air cleaners, when tuned properly they are similar in fuel usage to twin solexes, but give much greater performance mainly low end and midrange on standard motors
if you go the weber/dellorto route make sure you read up lots about tuning them correct venturies and jets etc before you buy
to overhaul twin solexes will cost about $70 to rebush throttle shafts (carby services in sydney do a great job) and about $70 for a rebuild kit do it yourself also a bentley or gregorys
workshop manual is essential to set them up correctly or try
http://www.ratwell.com/ he has a section on tuning twin solexes.