I use Speedotron and Elinchrom (Quadras). The two do not mix very well. If you use third party modifiers like Profoto softboxes, then it's easy. Just get a new speedring. If you want to put the Maxilite on a speedo VF202 head, well...good luck. If you want to go with Speedotron for the durability, reliability and power, then you want their Blackline pack and heads.
I use the VF heads because the modifier can be focused. Most heads and packs are interchangeable but there are a couple heads that will only work on the 4800ws pack. I have three packs; a 4803, a 2403cx, and a 1205cx. I like the 1205 cx for almost everything. It has very fast recycle...as in, I can shoot 8fps at 300ws continuous. It's comparatively light weight and it is looking to be indestructible.
I've purchased everything used and never had a problem with any of it. Some of the gear looks like it was used hard for a long time but it just keeps going. You will not get any of the bells and whistles of the Elinchrom stuff. No built in wireless triggering and adjustment, no built in optical slave, no 1/10 stop adjustment but, when you handle the Speedotron kit, you will know the difference! The word "tank" comes to mind.