Yup, the XR3135 is a turbo. I was looking for a turbo tractor too, since we're building at 7000', and I figured that a Tier IV tractor would clog up fast w/o a turbo. I was flip flopping between an LS 4145 and a Branson 4520R. Then I learned that Branson uses a cleanable DPF/Catalyst, and that you can evidently punch some holes through it if it's giving you problems, and the tractor doesn't care. That pretty much settled me on a 4020R, as I wanted the size, but didn't need the HP, or want to pay for it.
About a week before I was going to head up and sign loan papers for the Branson, I found a '03 MF1455 on CL. It's a non-turbo, non-Tier tractor and was half what the new Branson was. I drove 5 hours with money in hand and bought it. I honestly don't notice it smoke at all at 7000', and with a 55hp rating, there's plenty of power for anything I might ever want to do.
I don't know where you are in CO, but the LS dealer in Ignacio has an XR3037. VERY nice little unit. It's hydrostatic, but I don't believe it has a turbo. Seemed plenty powerful to me when I test drove it. He's had it over a year, so he's had to pay interest on it for 3-6 months now. You might get a decent deal on it... He had an XR4150 too, but wasn't willing to deal much last summer. As much as I wanted an LS, the Branson was a better deal, just not as refined. I also liked the mechanical engine, with add-on tier IV stuff, which was easy to defeat. The LS tractors are all electronic, which has benefits, but not for a DIY guy like me.
If you're near Fort Collins, stop in at Legacy Tractor Sales and see what TYM has to offer as well. They seem like well built tractors, and their service guy said the TYM's hardly ever come back. (He wasn't so proud of LS, thought he did say the newer LS tractors hadn't returned as much as the earlier ones.)
Good luck in your quest!!