Early in my prewar collecting days, and being a muscle car guy, I thought a less expensive way to obtain nice prewar was to restore them. How hard can it be?
It's not really, because they are small and easy to work with, compared to a car, but it is difficult to get colors to match properly, correct rivets, patina, etc, and usually, something is done differently than was done at the factory.
By the time you're done, you usually have as much money in the train as maybe buying one two grades better than what you're restoring. Problem is that restorations, unless done by a know restoration guy, are worth no more than an average train, and if you restore the wrong train, less. Imagine someone repainting a C-5 City of Denver in original two tone green. Some rare pieces may be better off with a restoration, but generally, that is not the case.
So, if you choose to attempt this, it can be rewarding, but do it for yourself. If you like the work, have the time, and the base train is at a reasonable price, go for it.
But realize that even though we like this stuff, a lot of it is quite common, and maybe the best approach is to be patient, look around, and buy a nicer example up front. And, if it's higher end, sometimes it is better off left alone than repainted.
This topic sort of bridges the Modern/Original tinplate topic, and as such, I am sure there are different views, however, if you look at ebay, and auction results, you'll find that restored pieces, generally don't retain value much beyond a C-6 original, and actually could cost at least that much if not more to do. So in essence, you could buy an original C-6 or C-7 piece and be done. If made in repro, the same would hold true. You just can't do a restoration for the same cost as you can buy a completed one for, so it's hobby value you get from doing it.