Start with a GL type unit, these are the safest home built reactor, they can be upgraded if and when you feel the need.
The easyest way is to start with a central heating water cylinder (immersion tank), but these are quite small if you do a reasonable amount of miles,
so you may want to go for a 205L drum with a cone on the bottom, jamesrl can make you anything from this to a full setup.
But as said above, get an oil collection round sorted first as this can be the hardest part.