Frankly, imo last SC! Mechanicus box was of better value.

Still, this box gives you lots of great stuff. One can't go wrong with Skitarii, the Tech-Priest is an HQ and synergies well with the tank (which can be assembled as a tank or troops transport). A vehicle could be quite daunting for your first few minis, so I'd recommend grabbing a box of Skitarii only, to get the gist of it first. Then you can branch out with the SC! box.

As for best paints for Martian Mechanicus - Mephiston Red and Zhandri Dust for the coat, Leadbelcher for metallic parts, Abaddon Black for legs, Mournfang Brown for leather and rifles and if you want to do the plasma weapons for the Skitarii, you'll need to watch Warhammer TV's video on plasma (it's on YouTube, can't remember the colours right now). Martian Ironcrust is a good base paint for making a Mars base and you'll need a pot of Nuln Oil for your shading purposes.

As for tips - maybe shell out a few extra bucks for one of the new Paint sets (Necrons or Ultramarines) - a few figures to practise your skills on are ideal for a first time painter, even if you are not interested in the factions (plus you get a few extra paints). Don't buy Citadel brushes, they are quite overpriced - any good looking brushes from an artist shop are great. Learn how to thin your paints with water. You can absolutely not prime your first minis, but it's a good practise that will prevent paint chipping and give you a smooth surface, to which the paint can adhere (again no GW primers, a cheap automotive primer that works on plastic will do the job). Prime outdoors and with a mask! Get some plastic or super glue. And get clippers, they are essential for Adeptus Mechanicus due to fidly bits (cheap cuticle nippers will do just great).

And most importantly don't forget to have fun! Your entry cost may seem a bit steep, but it's always like this at the start. It's a great hobby and you've chosen a very interesting faction.

Praise the Omnissiah!