Some more questions, and a few hints
The way you say about the changes required - it suggests it's a project home design. Are they basically trying to rip off one of these designs and tweak it? That's a bit of a no-go for getting a contractor in.
Have the proposed changes been drafted, or is it all at the verbal stage? Have plans been approved but not with the mods? Would the proposed changes be difficult to implement (structural considerations, site access, easements...)?
And how far have they got with anybody so far? I mean, have contractors come to the site and looked, and then not bothered to call back - is the site even pegged out yet?
As to sussing out potential builders - talk to a few of their past clients. A big call back problem is waterproofing. Ask if they've had leaky rooves or bathrooms. S**t like this happens often enough - hopefully not on your place. It doesn't hurt to check the builder's insurances and license are unsullied. No point aggravating future neighbours with a poorly controlled site (dust, materials, runoff, collateral damage).
And to attract the right builder, let them know you'll be 110% ready when they expect it. Have your (approved) plans with amendments ready, the site clean and pegged out, the finance organised.
DON'T change plans on the fly - this is guaranteed to piss people off, and can screw your home in terms of floor plan, plumbing or wiring issues.
And above all, be patient. The good guys are busy all the time. Anyone that has nothing on - that should ring a warning bell! Your turn will arrive. And never, ever ask "Will it be done by <insert holiday name>"? You're going to live there a lot longer than it whatever time takes to build.