Just additional to what digger has said
1. When Painting the surfaces using spray on paint - if you cant remove any parts that isn't supposed to be painted in that colour (e.g. windows, smokestacks), best cover it well with masking tape then while the paint is almost dry remove it carefully.
2. Of course get the missing parts for the cab and trailer. Good thing UM parts are easier to find on ebay these days than say pretender parts - based on the picture you just need well - UM head, waist plate/cab connector, chest plate, big and small fists, foot plates/ramp and his gun.
3. Make sure before you spray paint of brush paint that the surface is cleaned up from grease and residue - use babywipes as this is gentle enough not to cause damage but give it a good ol elbow grease if quite dirty.
4. Patience is a virtue, drying your coats before the next paint apps is a must. Otherwise you get a sloppy work turnout. 
5. Make sure to wear proper protective gear for your eyes, mask and do your spraying /painting in a well ventilated area. Also proper lighting helps.
Wanted AM partner Vanguard, Myclones Dirge, G1 Victory Leo, e-hobby Dark scream ( the black version), e-hobby Magnificus
Parts- AM partner Basher-side guns, G1 Actionmaster Elite Windmill's blades[I][B]