Studying on bustimes.org, Lands End to John O Groats is possible entirely on bus vehicles. Scania Enviro300s are becoming a common sight on the X98 (they tend to do the 08:55 journey out of Inverness arriving Thurso at around 13:00). That's right, a bus with no toilets on a limited stop 4 hour long route that is a good 100 miles in total distance.
Now, a journey allocated with Scania Enviro300s on the X98 is a stupid idea. The X98 in my opinion should be coaches only. Not only the route is very suitable for coaches (it's a 4 hour limited stop route), but with such a long 4 hour limited stop journey you'd want to have a comfortable coach with a toilet. The Scania E300s in my opinion should do the very local short routes like the 62/62D, 66, 80, 82 etc. (where a coach is not needed for such a short route). These are my opinions btw, but whoever is saying the X98/X99 should be coaches only I fully agree.
I think the X98/X99 should get B11RLE Plaxton Panoramas (like Glasgow X76) or Van Hool Astromegas (like the Oxford Tube), because not only you have a toilet with luxury coach seats, but then you can enjoy the beautiful scenery the Scottish Highlands has to offer, especially north of Inverness. The only issue with that, of course, is the luggage space (as the Panorama is a double decker coach so there is likely to be limited luggage space).
But basically, after Inverness, you can get the 08:55 X98 to Thurso, arriving there at around 13:00. According to bustimes.org, it appears to be allocated with Scania Enviro300s.
Then from Thurso, you can get the 80 to John O Groats (also allocated with Enviro300s, but with ADL E30D chassis instead of Scania chassis).