Where and what model to get.

I am going to convert a 15 gallon sanke toped bud keg. Where and what model valve/spigot should i get to put on the bottom? Whare do i locate the spigot on the keg? I am going to work up to all grain soon but i will be doing extracts and partials for now.

There's a great howto on the green board for converting your keg.

As far as spigot and weldless fittings. Get a zymico weld-b-gone ... and a half inch stainless ball valve and a barbed nipple size of your choice (in stainless).

As far as what you do inside the keg. I did a zymico bazooka t screen and their copper adaptor with an elbow.

http://www.zymico.com/weld-b-gone/

you can buy weld-b-gones at northernbrewer and morebeer i believe. they even come in kits with everything you need spigot side anyway.

This may help

https://www.homebrewtalk.com/showthread.php?t=3903

How close to the bottom of the keg should i drill the hole? As far as i can, correct?

It depends If you are going the no weld route. If so you need to leave clearence for the lock nuts and the further down you go the more curvature you'll have, leading to difficulty with sealing.

If memory serves, Northern Brewer was the best source I found for weldless valves, etc.

Ditto to what orfy said....also keep some distance between your valve and the "vent" holes in the keg.