what to buy and cube first

Don't overly spend on one piece of equipment; try to spread your money around equally (ex. rather than cubing 1 thing to Unique with 15%, try to cube maybe 3-4 things to Epic with just 6-9%).

Basically the way I do it is set 2 goals
1) rank up (use cubes until rare->epic, epic->unique, unique->legendary)
2) lines of useful stats
If an item gets multiple lines of useful stats, stop, period. It's better to cube something else. If it gets a single line of useful stats, which is better than some other item, stop there too. Otherwise, if it's a single line that's the worst of all your equips (eg. 3% when your other equips are all 3% or better) keep going.
If you're at the point where you need rank-ups to improve, do it one equip at a time, and once they've ranked up, get them to 1 useful line that was worth the rank up (eg. 9% stat on a unique) unless above goals occur sooner.