1. You seem to have quite a bit of horse experience and common sense. If your instincts are telling you to get the X-rays and you have an extra $700, get them.
Were it me, based soley on what you have said, I would not get them. Others may have a different opinion.
You can always call the vet you will be using for Roo and ask the question of him/her and what the fee would be.
Also, I keep forgetting this: Get a good equine chiro lined up. They work miracles. Find one that follows the up with a massage because that makes all the difference as to how fast a horse acclimates to an adjustment, if they need one.
I hate to say this but, in 20+ years of using equine chiros, it has been my experience women are better at following up with a massage. Men are too black and white; they do the adjusting correctly but that's it, they're done and sometimes my horses have been sore for a few days because there wasn't a massage, immediately following the adjustment
2. I thought Roo was taller than 15.2H before you said you're 5'8" - lol
I am 5'2" (and one half inch-lol) I had enrolled this horse in a formal metabolic study conducted by the Universities of MN and MI.
I had to measure Joker by the method they set out; it was not by hands, like most of us measure.
Mr. WTW and I measured Joker three times, according to their instructions, to be sure we were accurate.
Joker measured 15.3H. Meaning Mr. Roo easily tops 16H, IMHO
This is Joker and I. My legs are considered average length, you can see where my feet hit Joker, who formally measures 15.3H; admittedly sitting bareback also makes a difference. Joker is also a big barreled horse, like Roo; meaning that big barrel takes up more leg space than a horse with a more athletic build (I hope that makes sense)