2008 HST in grey with 135k miles on the clock, am I just asking for problems? What would you guys think this is worth?

Link to the ad?

Check the service history

If gearbox and all diff , t/c fluids were changed , and control arms, sway bar links, tie rod ends etc. were replaced, wheel bearings, air compressor, air springs etc. you should be able to run to 200K+ on engine and transmission. But if those parts (plus probably a few others) were not taken care of already - budget for replacement bills coming soon - could get quite expensive.

Check it is a genuine HST! There were two versions - factory order and dealer build from a kit BUT there are also lots of 2.7SE's dressed up with body kits bought from e-Bay dealers. It's a 2008MY car so also check it is not a "Stormer" S based special edition and was missing everything from the real HST except the body kit, leather upholstry and 20" Stormer alloys - ie it was the then base model with fancy clothes!
Value - nothing more than the base car it was based on - it could be a 2.7HSE, a TDV8 HSE or a 4.2S/C HSE
I have the original sales literature for them if you want to PM me an email address and I'll scan the pages for you. 2020 Pangea Green 1st Edition D240 New Defender 110 is here and loving it
2018 Melting Silver Mini Countryman PHEV - soon to be replaced
2015MY Corris Grey SDv6 HSE Dynamic, the best car I have ever owned, totally reliable only a cou0le of rattles in 3 years, now no longer in my care
Also in my garage is a 1996 TDi300 Defender 90 County HT made into a fake CSW

Agreed. A real HST should, I believe, have the rear e-diff. If that's missing then it's a "dress up" car. 2012 SDV6 - it's missing a couple of cylinders
2008 TDV8 - it was a labour of love and is much missed

I've just been looking through the original sales literature for 2006-2009 and the HST was only available with either the TDv8 or 4.2S/C motors, a TDv6 version was available with a dealer fitted body kit and exhaust system that made the car look like an HST but was marketed as the Stormer pack, this later became a special factory build limited edition car based on the S version of the RRS. Both the TDv8 and S/C HST's had the e-diff and adaptive cruise as standard fit equipment too.
So if your "HST" doesn't have the TDv8 or 4.2S/C engines or ACC and an e-Diff then it isn't an HST it is a standard RRS with the "Stormer" body kit pack fitted. 2020 Pangea Green 1st Edition D240 New Defender 110 is here and loving it
2018 Melting Silver Mini Countryman PHEV - soon to be replaced
2015MY Corris Grey SDv6 HSE Dynamic, the best car I have ever owned, totally reliable only a cou0le of rattles in 3 years, now no longer in my care
Also in my garage is a 1996 TDi300 Defender 90 County HT made into a fake CSW