Hi CJ,
I'm in a very similar situation as you: playing guitar a long time (and playing
slide very poorly for a long time) and only recently starting to sound OK at slide.
With that introduction, I believe the single best investment I made for playing slide was the three disc Brozman set (which I bought while Homespun was having a discount sale) -- well worth it. The second best investment would be a guitar set up as a slide instrument (high enough action and thicker strings).
And again, I'm in a similar situation to you as to not having many fingerpicking skills. I did get a DVD from Homespun called Essential Exercises for Fingerstyle Guitar. It does have a number of rudimentary excercises for begining fingerpickers (alternate thumb, index, thumb, index,... -- now thumb, middle, thumb, middle,...) and I have found that useful as those kinds of excercises didn't really occur to me. You might be smarter than me though, so I can't reccommed it as highly as the Brozman DVDs.
Regarding fingerpicking, I agree with everything that Barry says. I think one of my biggest detrements to my slide playing was wanting to use the flat pick in some way while using a couple of my fingers. My playing greatly improved around the same time as I put down the pick while playing slide. This had been recommended by EVERYONE, and I noticed that all the slide players I saw in concert and on DVDs were all playing with their fingers or fingerpicks.
John