Acrylics and model building: is a gloss layer a must-have before line detailing and washing?
Miniature painting is an interesting hobby, but as with any artistic technique, you need to practice and learn.

The answer to all your questions is: Do whatever gives you the result you like best.

You should first paint a small model that's not the centerpiece of your collection, or an old model you don't like best. Use this for practice and try out different techniques.

Any paint or varnish should be thined as much as possible but as little as necessary. On the one hand, you want the paint to be so thin that it doesn't overlay and hide small details, on the other hand, it must be thick enough to cover the area with pigment. I'd say a consistency between vanilla sauce and water is probably ok. If you thin the paint or varnish too much, you need to apply several layers to get the same effect. This is a matter of personal preference. Some people prefer a thicker paint that covers the underground in one single application, others prefer to apply several layers of paint to protect even the smallest details from being covered up. You need to try it out and find your own preference.

Black wash is the most simple way to create shadows and details in all the cracks and crannies. The disadvantage is that it darkens the whole area it's applied to. 


One solution is to dry brush to a lighter tone than intended in the finished piece and apply black wash over the whole miniature to darken it. 
An alternative is to apply a thin layer of clean water and adding the wash to it. The water dilutes the wash and makes the pigments flow better. The overall darkening effect is lessened, but you might have to apply several layers of wash. A tiny (and I mean tiny) amount of dish soap acts as a flow agent so the pigments flow into the cracs instead of bunching up on the flat surfaces.


But the primary purpose of gloss varnish is to make things ... glossy. It could be that the wash flows better over the varnish (similar to the thin layer of water), so it doesn't darken flat areas as much. Personally I don't apply gloss varnish before a wash, so I don't have first-hand experience.

In the end, it all comes down to your personal preference. If you like a glossy, metallic shine, apply gloss varnish. If you like a rough, dust look, apply matte varnish. If you want to paint a wet stain (maybe oil, fuel, water or blood), apply a matte varnish and then paint a glossy stain over the dried varnish.

A hint at the end: I prefer light primers (off white or light grey) because they make it easier to see small details during the painting process. I tried black primer once but it made the painting process more complicated because I overlooked small details and had to repaint them later.