You were right about that side locker.... It's rated for 2 X 16.2' horses and is shown with a 16.3hh loading no problem... My 16hh Welsh section D X Warmblood got into it a couple of times but really struggled to make the turn to stand herringbone. Just been to a 3 day riding club camp, had a fab time! Until it came to load, he went up the ramp nice as anything then just couldn't make the turn... Had to take the partitions out and tie him long ways, he travelled fine but so disappointed....
Personally (and I know others disagree with this) I'd much rather an Equi-Trek Star Treka. More room for the horses since they are rear facing not Herringbone. Though depending upon how your horse travels there is still the risk that they could climb onto the tack locker, but this exists for both trailers and I have not had an issue in the three years I have had mine.
They are also much cheaper second hand as they have been on the market much longer. I paid £5400 for mine (which was in good condition other than tyres and a couple of lights so well under £6000 all in) and I've not seen an Eventa come near to that. I've also not experienced a build quality issue so I am unsure how much of this reputation is deserved though I accept this is a sample of one.
You do need a car that is up to towing it. Air suspension is pretty much a must (some Mercedes ML/GL, some Cayennes, Landcruiser Amazon, Discovery, Range Rover) or a heavy duty pickup (Ford Ranger Wildtrack is the only one that is really up to it) due to the ~200kg nose weight. A low range box for maneuvering is highly reccomended (again an option on the ML/GL and Cayenne and should be standard on the others) and be prepared for quick clutch wear if you get a manual. If you want to carry 2 horses then a 3.5t towing capacity (also available in all of the above) is also needed since the trailer is ~2t unladen. Then again the situation is going to be the same for the Ifor I would have thought.