I'm in exactly the same position and have come close on a few occasions to giving in. To date with all parts and the engine build in about £12k in, I believe the build will still need £12k to finish.
This is more than just a single, this is bodywork, suspension, wheels, brakes, single turbo and an engine build. But it will be done to a very high standard and I don't think I would find a similar car for sale that I would be happy with.
Therefore I will continue. Mine has been of the road 2 year this month. I have made a promise it will be back in the road for summer next year.
Stick with it. You can't beat building your own car. You will never be satisified with anyone else's attempts.