Some of boot fitting is trial and error. I suggest looking for used boots at first and see what works. You probably will make fitting mistakes at first. Better to buy used ones you can resel for about the same money if you do make a mistake or the boot isnt quite right for that horse. Its pretty easy to find used Easyboots of all sorts and Cavellos. Not so easy to find used Renegades. People use them till they wear out.
Re: Boas. Yes they have some positives but there are some serious negatives to consider also. I had a set I used for a long time on my SSH gelding. They are not made to ride alot of miles in. Id say they are better for rehab as stated above and minimal easy trail riding. Too clumsy for rocky technical trails.
The negatives would be sever bruising from accidental over tightening of the dials (seen this several times) difficulty changing broken cables - its almost impossible. Rubbing at the corners of the pasterns even with pastern wraps due to design. Weight - they are very heavy for a boot and clunky. They are only sold is pairs last I checked. Kinda a PITA if you loose one.