IMHO and from my experience, it depends on what you want from the platform.
I started with a dumb trainer, speed and cadence sensor and cheap 2nd hand wheel on trainer (r1000 for the sensors and +-r700 for the 2nd hand wheel on).
It was OK to get started but it doesnt take advantage of the platforms, no resistance, you can take part in events/races but result might not count for results/Zwift Power, etc. and for me it became boring. In your case you would also need to swap the MTB tyre to a slick. call that r2000 if you shop nicely.
Having said that, if you cant find a good deal on a 2nd hand trainer then this option is not cost effective.
no point in buying a dumb trainer for 2-3k and then you must still buy the sensors, that result in an below average experience that will not grant you access to the proper world of zwift training and racing.
But if you dont care for racing or erg training that will be your cheapest way to get zwifting.
1 step up would be a wheel on start trainer (wahoo kickr snap or the like, +-r6000).
I found myself a 2nd hand snap 2 but I also found the resistance to be "weird" (too clamping). You will be able to get racing (you will need a HRM too) and the experience is far more enjoyable with resistance.
From there a direct mount is the best option, and if I had to do it all over again, i would go straight to a direct mount.
there are some good deals here and if you patient you can find one for +-r10 000.
The resistance and flywheel of direct drives feel more "real" and the sensors became irrelevant in my case as the kickr core transmit power and cadence. also, no tyre wear or black *** all over my floor.
ERG training is great too.
if you just want to turn your legs while you see a guy on your screen pedaling, just find a really cheap 2nd hand wheel on trainer, cheap slick and get some sensors. done.
If you think you will get bitten by the indoor bug and want to really train and take part in racing and maybe join a team to do WTRL and other series then bite the bullet and go for a direct drive.
The experience (to me) from a wheel on to a direct drive is worlds apart and worth the price tag