Good choice on the rifle. Marlins are great shooters for not a lot of dough.
I have two 22-specific scopes: a BSA Sweet 22 and a Weaver 2.5-7x28. The BSA has more neat features, such as the turret that is calibrated for the bullet drops of several different bullet weights, in my case, 36, 38, and 40 gr. So I zero it at 100 yards, and I don't need to mess around trying to remember bullet drop at other distances. I just dial that turret to the distance I'm shooting at, and BAM. The reviews on OpticsPlanet were great. But I admit, I've only shot it at 100 yards so far.
The Weaver is a lot smaller scope, but optically better. It looks more appropriately sized on a 22. It is every bit as good as my Leupold, and was only $160 or so.
With 22LR for small targets at longer ranges, 7X is about the minimum you'll want to go on the high end. 9X would be better, and 16X better yet. But then, the scopes are getting huge. Will that bother you, or are you willing to have a big, heavy scope and get the maximum performance out of your gun?
You're getting a 22 Magnum, which will reach out to around 200 yards, I hear. You'll want a bit more magnification than 7X. I definitely wouldn't go smaller than 9X. Again, 16X will be better.
One thing though. Before you decide on a 22 Magnum, look at the ammo cost compared to .223 Rem. The .223 Rem is a much more powerful cartridge, and ammo doesn't seem to be too terribly much more, especially the bulk stuff. It is a true 300 yard caliber, and I bet you could reach out to 400 or 500. BSA also makes a Sweet 223 scope to match with that round's trajectory. The quality is good. Mine, a $110 model, came with screw on caps, a sun shade, and a cleaning cloth. The Weaver, while optically better, came with none of this, which is a bit annoying.
A centerfire scope will be fine, but will look a bit big on an XT-22. If you're shooting at less than about 50 yards, you'll wish you had a rimfire scope.
Leupolds are nice because they're US-made, and have the best warranty in the business. It is just hard to pay that extra $70 or so compared to the Weaver though... Weavers are made in Japan. BSA: China.
Think hard about the .223 though. It is better for targets, varmints, AND plinking. You can get into hand loading for it (it isn't hard) and make VERY accurate ammo for a bolt gun. Savage makes some very nice ones for a fair price. Not quite as little as a Marlin 22 WMR, but remember that it is a good bit more gun, too. Same ammo as the beloved AR15, so bulk ammo is everywhere for cheap.
urchase online, and find bulk price.I like the now discontinued pmc 40 gr sp/hp,but the Feocchi seems ok. I don't ask for sub min of angle as I use for varments,and if it were legal,the 40 gr sp would work quite well for neck shots on blacktail deer. Report quiet,150 yd range,30-50 gr bullets available,win supreme is great varment but hard to find and costs more.
