I'm playing a free spirit of the possesion tradition and decided that the "stock" body that I'm going to use is a Plasteel Homunculus that can hold a force five spirit. While it does have a cost in materials for making the body (Maximum Force x 2,500 nuyen) it doesn't give an availabilities on it, neither for starting off play with it or acquiring it in-game. I was hoping that I could get some opinions on that.

I sometimes use the Availability value given for hardware in the Hardware Upgrade Costs Table (SR4A, p. 222) when I am dealing with a piece of equipment of unknown Availability. So, in your example, the homunculus would cost 12,500 nuyen, and I would use the sum of Rating and Signal from the table that is equal to or greater than that value, which is Availability 16. This would require the use of the Restricted Gear Quality from Runner's Companion.

It's a rough estimate, I know, and doesn't take into account the radicals spent on the homunculus, but GM's often appreciate the player doing a little legwork like this, and yours might just go with it. Especially if you buy the radicals as well. Alternatively, she might just tell you it is actually Availability 20 because of the radicals, which would still work.

Not particularly up on the SR4 as sunk deep in SR3 at present but you may wish to re jig the focus rules at character creation as a vessel body is much the same thing.