I highly recommend the Mini A7 as a starter especially if funds are limited. They cost about $20.
Mini 2.0 with Killer 705. This is from Greece and is not a budget RBA but it will fit any wells or catchcups that a standard diameter 510 or carto will fit. It's original design is to be used in carto tanks but it is a most excellent BF RBA and can be blind squonked.
Hornets on Mini 1.0s except for Purple VVG.
Killer in picture needs it's bottom adapter put on or it is way too airy. (note big holes)
For some size comparisons.