I have a Gammill--welcome to the world of Long Arming! I have many add-ons (bought that way, but no hydraulics which is what I'd like to be able to afford). Here's what I'd suggest: maintenance kit; stand alone bobbin winder(and I buy 2 cones of almost every thread so I can leave one on the stand alone while quilting); extra bobbins; extended table; micro-stippling handles if you like the look of denser quilting; light bar on the machine (the overheads are nice, but where you put it may already have nice lighting so all you need is the intense light close to the needle); panto handles if you think you will do them--and most of us use pantos at times; you don't say what size table you are getting--but I would suggest a 12' if you have the space--that allows you to quilt most anything (I have a 14' and rarely need that lenght). And the laser light.
What I'd not buy--the Gammill guide (I don't have and everyone I know that has it says it's hard to handle); casters (you likely aren't going to be moving it often enough to warrant them--mine came with them but never been moved--it's too heavy even with casters!); the start up kit--you can get all the supplies other sources and unless they are giving you a wide assortment of thread weights and needle sizes, you will want to do a little experimenting to see what you like best. Several on-line stores carry lots of Gammill stuff--Linda's Electric Quilts, Kingmen Quilting, etc. You can get a drafting stool for $99 at most office supply stores and you really don't need the guide for a stool as most have locking casters and you can lock a wheel and not worry about flying away. The Gammill Guide jr. is just a longer straight ruler with a handle on top--I like my Linda Taylor straight ruler much better. And Gammill's are thristy machines so you will need oil--but again, that's easy to get too.
hold off on zipper leaders--I bought those and then decided I was still just pinning (I didn't like to sew the backing to the zipper leaders)so I ended up switching to Red Snappers--which I love.