I don't see why. Something like Amnesia was made on an indie budget where 250,000 sales was an amazing success.
Yes, and they did their engine themselves. Today you can easily license a capable 3D-engine for a very small amount of money - for instance, Garagegames' Torque 3D seems to work well and a "full source professional studio license" for that will cost you just $179.
Of course, an engine is only part of the equation and the development of 3D-models, textures and so on can potentially cost a lot of time and money. But I think that a Tex Murphy game could live with some rather basic stuff in that department. Just look at some of the adventure games from Telltale Games. I think they're great and they certainly seem to have been selling fairly well, and they have some very basic models, textures and effects. I don't think more is needed for an adventure game. Of course Tex Murphy would need less of a cartoony style, but gritty can be made on a small budget too.
That said, I don't personally know much about how important the 3D was for exploration in the later Tex Murphy games - so far I've only played the first two. But I loved the 3D in the first one, just for the nostalgia. Hugely impressive, if rather dull, flight simulator for such an old game.