The toughest part usually is to get a good set of 3-way drawings / blueprints / line drawings. This is usually impossible for most helicopters, only good one's I have, are for Bell 412, and that's only because Bell put their Line drawings into "Product Data" brochure available in pdf on their website (insane !). For example the S-58T I'm working on, was build upon drawings of Westland Wessex, with only an outline of S-58T nose, so the nose on that model IS wrong, and in need of being corrected. If I had personal access to the bird, I would just take a ruler, camera, notepad... and do a set of accurate blueprints myself.
Tried to do it with S-61N using available pictures, but couldn't finish it... I snapped after changing hundreds times position of a single line, as my calculations formulas were constantly off. When it looked like I had CTD I pulled the plug on that project... not worth of all the hair pulled out

I have a lot of documents on different helicopters - pilots handbooks, maintenance/flight/training manuals. And I love the flight sims, but there is only a handful of good machines there - for FSX there is really only one - the Dodo B206B3, and even though a great piece of modeling and coding - the machine itself have limitations. So if you want something be done right - do it yourself. But there is a problem. The reference materials are usually not good enough, or lacking in some areas.
For example - I have NASA paper on physics modeling of helicopter flight based on Sikorsky S-70 model, great stuff... if you're coding a flight model for simulation - not when you make a machine that is using the existing one. Same in many different areas. So I actually made a spreadsheet to see what I could do, and what I shouldn't. There are 25 machines there, 22 characteristics, the ones that have the highest, have the top places... the first 5 are: Bell 212, Sikorsky S-61A, Bell 412, AW139 and Eurocopter AS365... non of witch I would give my "vote of confidence" as in every one there is something very important lacking. The funny thing is, the S-58T is on place... 16th. So yeah... I need a lot of help on that one.
And no - the Mil Mi-14 is nowhere on that list, because, I have everything needed to do it, including people to flight test it, BUT... it's just to close to my heart to try, as I know it would take me forever, as everything would have to be absolutely perfect, and that's just impossible.