I've built the Evo3 and Evo4 (Haseqawa) but not yet the 5 or 6. The best advice is just to follow the Tamiya instructions they are very good. Test (dry) fit each part before glueing. Carefully remove the mould line which will probably be on many of the parts especially the drive shafts. You may also find fine mould lines on the front and rear quarter panels near the front and rear lights. These must be removed as well or the body paint will show them. The wheel attachment to the brake assembly is often a weak point so decide whether you really want the car to roll on its wheels. If yes then be very careful with the glue application, if no then be generous. If you plan to display the model with the front wheels turned then don't foget to glue the steering wheel at an appropriate position.
You could really get away with matt black, silver or aluminimum. If the interior is white then if you really can't afford the paint then the interior could be left unpainted (I've done this before) but make sure the seats and dash are painted well and the roll cage (as you mentioned WRC I presume its the rally car) is glued well and any mould lines are carefull scrapped/sanded away. The detail on the dash will tend to draw the eyes attention away from the rest. The body can be painted any time but do not fix the glass (use a white glue NOT a cyno type glue when you're ready for this), headlights etc until you have the body painted, also obviously do not fix the completed chassis to the body until its painted.
The decals work well with Micro sol/set.
The Tamiya Evo's are fabulous kits and will produce a splendid model. Take your time, follow the instructions and you'll produce a great result.
Kia ora,
Mark
"Time flies like the wind, fruit flies like bananas"