Either or for the turnouts...personally, I would build them first since handling them will get them gucky as you build them. Once built, gently do what you must to get them burnished. If your solders are solid, you should be fine.
I don't know what stall mats are, but if they are solid and won't let small screws or grab irons and stuff fall into them (i.e., below their usable surfaces) they may suffice. As you have surmised, seeing is a problem on layouts...seeing to get rolling stock and engines properly seated on the tracks, enjoying the motions of valve gear if you are a steam nut, and being able to see fairly easily where a dropped item has fallen. Dark, non-motif or textured flooring is best.
If you have not thought much about the lighting, and depending on your age and visual acuity, I would strongly suggest track lighting (Home Depot carries an inexpensive brand that I use...8' tracks that you can use to hold several pointable fixtures that take GU10 50 watt halogens. They are warm (!), even hot to handle when they have been on for more than 5 minutes, but they sure light up your viewing pleasure. My point is that if you decide that your handiwork is rather pleasing to the eye, and you'd like to post some nice imagery here, say on Weekend Photo Fun, you will have wished that you had a way to direct lighting to best show your various scenes. The system I am talking about, from HD, allows you to reposition the pots/fixtures as you need them..albeit on their track. I have two over my 13' X8' layout, each with five fixtures, and I can get good lighting with the twin tracks set parallel and 4' apart.
You have to be careful with flourescents...they can be very cold in colour temperature, and they do not direct light to any great extent. They also give off UV, not unlike the halogens, and that can have a deleterous effect on the ground foam and its colour over time.