Take it from some one that has painted several thousand of those GW figures and gaming in general.
Get in the habit of painting one unit at a time. Units just need to be consistent and unless you are entering in a contest only need three colors and look good at arms reach on a table top. (again, unless you are going the extra mile for contests and tourneys) But you extra effort on "important" or eye catching pieces like character models or "large based models"
If you can paint on unit at a time you will achieve a consistent look in that unit, the variation between units is not noticeable on the table top between units that is a result of your painting improving provided that the color scheme is similar though out your force for a consistent look of the whole.
The boxed starter set features easier than normal models to get you playing quickly. If you have any doubts about what to add next. TROOP choices. They are even MORE important in this latest incarnation of the rules of 40K. I usually build a list around 3-4 troop choices then add fast attack and heavy support as needed with the left over points limit that you agree to with your opponent.
I then to build a list around a core of troops, a "Calvary unit" to go grab objectives (this does not need to be fast attack, I often use APC mounted troops) then a "lighting rod" This is something big and scary that my opponent is afraid of. Literally, find something in the army list that everybody can't ignore or help not to throw dice. This is the unspoken role of a Land Raider, of greater Daemon, or Monolith, a LOT of players will shoot at it just because, and while they are doing that, your troops are grabbing objectives for the win. In 40K never lose sight of the victory conditions.
Your dread and termies full fill the role of lightening rod quite well in small points games.
This should help you figure out what to buy and paint NEXT, although don't think you need everything on GW's wall either, it just depends on what points value your gaming group plays at. The black reach box is fairly old, (I am not even sure which rules set is included in it :0 ) But it is a decent start towards a ork or space marine army. It has a character model, 1 full troop choice or 2 minimum sized ones, one dreadnought (elite choice) and 5 terminators (another elite choice) for space marines. I think (I am not a ork player) that there is a even correlation of forces for the ork side. But I would add another 10 man tac squad to the marines and a large mob of boyz to the orks and maybe a truk.
The game get really fun when the armies are much bigger. 
