I don't know the answers to all of your questions, but I've recently purchased a TT bike (I also have two road bikes). I'm something of a newby to cycling myself, but I have a coach who is an excellent & experienced time trialist and a fitter who is extremely well-respected internationally and whose 'thing' is aero testing. So from them what I know is that TT bikes are not as adjustable as road bikes, it's possible you could buy a bike & never be able to make it work for you. Sorry, I had a pre-purchase fitting & was given a shopping list of appropriate bikes, I'm not sure how you'd decide on which bikes you'd consider otherwise (not saying you can't buy without a pre-purchase fit, just that I personally don't know what the key measurements are).

The shopping list I was given was: Fuji Norcom Straight, Cervelo P2 or P3, Felt DA1, Look 956, BMC Timemachine and Trek Speed Concept.

Of those, the 'bargain' bikes were the Fuji and the Cervelo P2. I wound up buying a Fuji a month or so ago, it came out to $1600ish for the entire bike. They are on sale right now through Performance Bikes (Internet). The Cervelo P2 would have cost me $2200ish if I paid cash. The P3, Felt, Look, BMC, and Trek would have come in around $4000-$7000, more than I wanted to spend.

Since my intention is to race the bike, I knew I'd be replacing lots of stuff on it- wheels, saddle, crankset (I would be putting acrank-based power meter on it), bottom bracket (for compatibility with my power meter). Once I rode the bike a bit, I installed Di2 electronic shifting & bought better aerobars, two things I knew I might do but wasn't sure. So keeping the starting price low for me was key, it might be different for you if you're not immediately swapping out components to the extent I did.