Are you building it yourself, paying to have one built, or buying one already built?
If you're having someone build it just don't, you're going to get scammed.
If you're buying one already built be very careful, there is a lot of garbage cars out there. Make sure to inspect the chassis, suspension, body reinforcement & mounting, door hinges & glass, everything. Most "replicas" being sold are garbage that need a ton of work to fix.
If you,re building it yourself know that these cars are not easy to build, it takes an incredible amount of skill and time to do it right. If you don't have much fabrication experience you're in for a tough time, be prepared for it to take many years and a lot of money to learn, even then you'll probably end up with garbage unless you have a really good mentor and a lot of patience.
As far as budget that depends on how good it is/you want to make it. Expect it to cost more than you think. But seriously if you're not a fabricator I recommend avoiding these cars all together.