These are just my opinions, but here they are:
If money doesn’t matter, buy something you are excited about. Especially if one buys used, you can generally get vast majority of what you spend back. Good instruments appreciate in value. If you buy something you are excited about, you will play it more, you will get better faster, and you will enjoy it more (which starts the cycle over again).
Try many guitars to see what feels good to you. If you are a smaller sized person, a smaller guitar body and narrower nut width might be more comfortable. With that said, if you wanna play bluegrass, bigger guitars are generally favored for their power.
Here’s a couple guitars and what you can expect to pay that I have personal experience with and think are good buys.
Seagull S6. They can be had for just over $400 new. The thing that impresses me is that every Seagull I have ever played has had a decent setup (action and intonation). They play very easily for a “cheap” guitar.
Martin D-18, the 2012 model. They made some changes this year for the standard D-18 and I think it’s a great guitar. They can be had new for about $1800. Used ones aren’t generally available yet.
Martin D-18 GE I love this guitar. The sound is that prototype Bluegrass sound. Used around $2000 and up (I recently bought a sunburst one for $2400) and new for $3000. It’s a nice guitar, but not so nice that I am afraid to take it out to play.
You’ll see Ben playing a McPherson in some of his vids. It is the one with the unusually located soundhole. I don’t own one, but I have played several and given it serious consideration. The guitars are very innovative. They have a unique sound, almost like playing through a scooped EQ. They play fantastic. They can be had for $6000 and up.
If you really want to plug in, I have a Martin GPCPA3 I really like, and the Taylor 414CE is a workhorse. Both can be had for about $1300 and up used and $1800 new. There are also dreadnaught versions of both.
There are some other great possibilities such as Eastman and Recording King, but if it was something for my Dad to play bluegrass, I would probably get him a standard Martin D-18. Some will say that’s way too much to spend to start, but as I said before, I can get most of the money back out of it even if I buy new.
Have fun shopping!