If I understand the TruTrak docs correctly it is not like the OBS mode because it is not related to any particular waypoint.
AFAIU it workså by steering the aircraft so that the actual track angle flown corresponds to a number the pilot has dialled on A/P.
I’ve just read in the USA that TruTrak and Trio autopilots follow GPS flight plans only. No VOR/LOC/GS signals, so they cannot fly an ILS, and it isn’t clear about LPV.
They do fly LPV coupled to a WAAS IFR GPS quite nicely.
Can it fly an ILS?
It is not certified to fly an ILS ;-).
But does it? Post 32 suggests not.
The certified trutrak autopilot is the vision and it is not able to fly an ILS. LPV are OK as precision approach.
The trutrak autopilot that is able to fly an ILS is the sorcerer. At this time, there is no certified version of the sorcerer, only an experimental one. And I saw no plans to certify the sorcerer at this time.
The trio autopilot is also not able to fly an ILS. Only LPV.
The Q would be: how many people who read these “look how much innovation there is once certification is relaxed” headlines are drawn to paying for this “certified” stuff without realising it is, in most European scenarios, largely useless 
The ILS is the gold standard here for IFR – unless you fly only between LPV airports, or want an autopilot just for keeping the plane straight enroute.
Probably quite a lot… One guy I know of aid an estimated 40k for a full new panel in his IFR tourer and when I asked him if he got LPV, he wasn’t sure Another paid over 30k with a belief that it will get him “full IFR”.
It depends on how much of a full disclosure of final aircraft capability the installer makes in his quotation. Most people having work done don’t ask that question.
It looks like after underselling the entire market with the G5, Garmin may be having a repeat performance with GFC500: it is certified for ILS, it has STCs for C172, C182, PA28 and BE35 with more types coming up this year, and it costs around 8k including the servos and the STC.
Installed prices from real world
GFC500 + G5 (mandatory) + install = 8k + 2.5k + 3.5k = 14k
Tru + EAA STC + install = 5k + 0.1k + 1k = 6.1k
Trio + STC kit + install = 3.5k + 2.5k + 1.6k = 7.6k
The double price for Garmin gives you all like the others plus radio guidance ILS/VOR/LOC and the G5 instead of a HSI.
Your choice to spend the premium factor of two for a rapidly fading away technology and a little glas screen.
