This is probably why so many builders focus on one make, model, and/or manufacturer. You build familiarity so that you know what works and what doesn’t, which helps avoid mistakes that blow the budget. You also know what the parts you need really cost, so your budget is more realistic from the outset.
I did a modest restoration, and not a conversion, on a car. It started off being a fix and flip and ended up being a “full restoration.” Part of that was because I didn’t assess the car as closely as I should before buying. Part of it was wanting to do the car “right” knowing that I would be selling it once it was finished. I wanted the car to be roadworthy and reliable (which is what you talk about frequently on the Fairmont project). So I engineered fixes to common faults. Not only did that take the car over budget but it also made it take about twice as much time as I originally envisioned.
In the end, time was my friend, as the market for this car was on an upswing and I was able to sell it for pretty much what I had into it…not counting my time. I consider myself lucky, but by the same token, I had an idea of what the maximum sales price would be once the car was completed and worked hard to stay under that number. So, to some extent it is all about discipline.
Where am I going with this? If you want to build a car because you want the experience and want to put what YOU want on the car, then build it however you like and accept that budget really just isn’t a prime motivator. Which is o.k. — if it is your car do with it what you want and make it something you will enjoy.
On the other hand, if you want to get out of it what you put into it (i.e., to sell the finished car), avoid the “exotic” parts and plan your build around who you think the ultimate buyer will be.
That’s my $.02.