Chris, your profile says your current type is DHC8, that being the case - either Q300 or Q400 series - you'll be aware of the capabilities of that type and its nav kit. Your turboprop type's single FMS installation must be certified for B-RNAV operation (more likely it is already P-RNAV certified), it is very unlikely to be a single-source FMS (ie, it will prioritise GNSS/GPS, but will also receive DME and perhaps VOR info).
'Map Shift' error only happens to single-source RNAV equipment, most systems will 'flag' system issues and position uncertainty messages. 'RAIM' is only relevant for GPS-dependent procedures, ie where the capability to fly a given procedure depends on the availability of valid GPS signals (principally, RNAV/RNP approaches). You need to get hold of and study the Handbook/User Manual for your FMS.
Any B-RNAV equipped/approved aircraft may be flown using the FMS for steering guidance above MSA. European aviation authorities (incl. UK CAA) accept that (even public transport) operators fly conventional departures (from take-off) and arrivals using the FMS ... PROVIDED the procedure is designed to be flown using VOR/DME/NDB aids, and those aids are used to confirm FMS track-keeping accuracy. It must be understood that the conventional navaid information will have priority over FMS information if any conflict arises. It is not permitted (as a B-RNAV operator) to use RNAV to fly departures and arrivals that cannot be flown solely using conventional navaids, or that are labelled RNAV Only.
Subject to the above, no reason at all why you should not use the autopilot coupled to FMS for steering the aircraft, this will give you the capacity to monitor conventional navaids fully and revert to heading or VOR coupled nav if necessary.
Try this for good straightforward guidance -
http://www.ecacnav.com/downloads/P-R...20notices).pdf