The main thing about treeless saddles is that they're NOT a solution to every saddle fitting problem!! Sorry to state the obvious, but its amazing how many people think it is; all you do is to bung on a treeless and hey presto off you go.
It ain't that simple. The MOST important thing, if you're committed to going treeless, is to consider what you'll be putting underneath it. Sorry if that sounds @rse-about-face, but you need to bear in mind that this is absolutely vital as you'll have no tree in place (obviously!) to support your weight on the horse's back. Proportionally, you'd be wise to spend say half of your total budget amount on your treeless saddle pad (and don't believe any websites or individuals that say you don't need one), and allow the other half say, by proportion, as £££ for your actual saddle.
If you're a first-timer, then have a look on e-bay as there are some nice saddles which come up secondhand, but DON'T repeat don't be tempted to buy the cheapie ones made in India; or anything that hasn't got the maker's name on it. If they can't be bothered to put their brand on it, then it ain't worth buying IMO. There's a lot of stuff on e-bay to not touch with a bargepole, BUT there are some very nice treeless saddles on there much cheaper than you'd get new.
The advice I was given was to go for a good make like say Torsion, Barefoot (the older versions are like hens teeth, the new ones are awful apparently - which is they there's so many of them on e-bay now!); Libra, Freeform, Heather Moffatt (always retain their price).
OR you could try to find a company that offer you a trial period........ do some research as there are those that do.
But bear in mind that treeless saddles tend to do two things (1) they have a far wider twist than treed saddles which if you've got lower back pain/hip issues, might not be helpful, and (2) you'll find yourself riding "higher" on the horse's back than you would with a treeless saddle. Also you'll find you feel the horse's movement much much more, fine if you've got a nice comfy gait but bear in mind you as rider will tend to get more fatigued more quickly. These are just my experiences. I bought a lovely Libra leather treeless on e-bay, they don't make them in leather anymore, and its been very useful as a spare/backing saddle. But I am sooooo much more comfy on my lovely treed saddle with knee rolls - total bliss
Nope, I wouldn't go mad about treeless TBH, but hey each to their own. They have their uses, and have their fans.