Panteras came with a 5.7-liter Ford/Cleveland V-8 of 300-up horsepower. Engines were used in hundreds of thousands of dependable passenger cars and are simple to work on, as there were NO electronics at all. Parts are commonly available. The engines were also built in Australia & were the preferred performance engine there even today, 25 years after they were discontinued. Expect about 20 mpg around town and slightly more on long road trips.
The 5-DS-25/xx ZF transaxle IS a problem. Panteras, Mangustas, GT-40s, Maserati Bora and indeed most '70s-'80s mid-engined Euro sports cars all used the ZF, but that was 40-50 years ago. The design was sold in the late-'70s to a private shop in Southern California along with the license to mfg parts, so there's essentially still a single source of verified spares world-wide. A fully reconditioned 5-speed ZF will be around $10,000 USD. A number of ZFs have been rebuilt via cannibalization of usable 'pre-owned' parts from damaged assemblies. Repairs can be from all skill levels from OK to the seriously bodged. At the price, you really want to drive the car to assess a ZF's condition.
The DeTomaso Drivers Club in England would be my first local point of contact. In the north of England, try Roger Brottan at http/www.theepointfour. co/uk, tel 01226-719443, or in the south, www.woodsautomotive. co/uk. may still be a good url. Or lurk here- there are some bargains that turn up now 'n then, within your budget. But the GT-5 (or GT-5s) is a rare car and well-done conversions none too common.