Hey I'm not trying to burst your bubble, but I think you budget is not realistic. I would not personally let my car go for anything under $30 grand, and even then I would have to seriously think about it. Just to give you a point of reference. This is a link to a stock z http://www.ebay.com/itm/Datsun-Z-Series-/122034583640?forcerrptr=true&hash=item1c69d3f858:g:l6EAAOSweWVXeQxR&item=122034583640 its buy now price is $36 grand.   
  
A 2jzgte with manual transmission is almost 10 grand.  http://www.ebay.com/itm/JDM-94-98-Toyota-Supra-2JZ-GTE-Twin-Turbo-Engine-6-Speed-Getrag-Transmission-BOX-/141951601218?hash=item210cf98a42:g:g9AAAOSwWBJXBXa-&vxp=mtr 
  
  
I wish you the Best of luck finding your car! 
  
EDIT: 
  
Just to add, here is a link to Hagerty values for the 240Z. I realize its for a stock Z but gives you a point of reference. 
  
https://www.hagerty.ca/apps/valuationtools/1973-Datsun-240Z 
  
  
The following are for 1973s: 
  
Concurs  $64,000 
Excellent $41000 
Good $20,000 
Fair $9000