First question is that how do you understand if a lens is of high quality and will produce quality DOF (better DOF).

I'd like to explain DOF as not dependant on lens quality but dependant on aperture number (f-number). Usually low f-number lenses are expensive like 85mm f/1.4 is pretty expensive than 85mm f/1.8 costlier. f/1.4 lenses are also termed as better quality lens due to their internal structure.

Let's try one thing. Keeping ISO and shutter same, shoot at f/2.5 with 85mm f/1.4 lens then shoot with 85mm f/1.8 lens. DOF will remain almost similar irrespective of the cost of the lens. But if you want to produce even better DOF with either lens, you will have to shoot at f/1.8 on 85mm f/1.8 lens and shoot at f/1.4 on 85mm f/1.4 lens. Now you will find you the background is very much smooth in that image which was shoot at f/1.4 on 85mm f/1.4. So the reason is lower f number produce better DOF.

DOF is not the only reason to buy expensive prime lenses. There are other factors as well:

1. f/1.4 lens will be fast than f/1.8 lens.

2. Ideal for low light situation.

3. Better quality image.

If you find above points reasonable to invest large amount, can go with it. I don't think so until you buy lenses, can try new lenses at shop. Though you can get them all on rent for a day to experiment if you like the performance. Then you choose to go with.

If I'm hobby photographer, I'd prefer to go with f/1.8 lens as it saves a lot.

Reply for more queries on the topic.

there are 2 aspects that create DOF.

1) Aperture of f/number

2) Distance b/w you and your subject should be less than the distance b/w your subject and the background