I think you must first form a buget just like any other project. Include a best case and a piss off the wife a little case. Best case in the USA would be finding a older GTD that needs to be rebuilt but is all there and with current tags. My guess here would be somthing on the order of 30-40 grand and then another 10-20 all in. This will get you a very nice car with some or all of the upgrades.
Then you could do a RF for example like Ron, this can go more towards the pissed off wife zone but you would have a newer design with many upgrades installed allready.
Big options are really weber style fuel injection, pin drive wheels, and choice of gearbox. Oh and paint. If you can find a completed car painted the way you like it then I think you could save a lot of money. To get one painted is a LOT more that you think.
I have said this before but the first option to settle on should be the choice of gearbox. My opinion would be a G50 porche type. This is the only modern box that will settle most if not all the gearbox issues, strenght, ratios, LSD, replacement and avalibility, without going over 5 figures. Still you could be close to that all in.
In my opinion you will have a hard time finding a "I give up" new type car cheap. The kind of people who start these kind of cars have done their homework and know what they are worth, They also know that they can sell a RF build that is not completed for example, very quickly for a price that will get them the sum of the parts at least. Just a matter of days really.
Begin your reading and study bodywork issues. The newer cars will have a lot less fit issues. This CAN be a VERY big deal.
Lastly I think that the quick build option that RF has is going to be the future for these cars. Within a couple of years that will be the only way to buy them from anybody.
You should know that these cars are not cheap. Be perpared to spend 60-80K before you are done.