If you think you'll be grilling a bit, I'd consider making the jump to the Genesis. Having the sear station is great, plus the build quality is a bit higher, with the Spirit having quite a bit of plastic. (Go into Lowes and you'll see what I mean.) They're both great grills though.

Brush is necessary. Some people have strong feelings about not using a wire brush because the bristles can supposedly fall out and get in your food. Never had it happen, but up to you to decide. You may want to opt for a nylon brush.

Tongs are a must. So is a good spatula if you want to grill things like burgers. Really for everything but the thermometer I'd just get a kit like this Cuisinart one for $20, as it will include all the basics (brush, tongs, spatula) plus a few other things like a basting brush, corn holders, etc: http://www.amazon.com/gp/aw/d/B002XQ...E-L&ref=plSrch

I've seen people lean both ways on a grill cover. Reason being a grill cover can actually trap moisture, which causes rust, whereas a grill will dry out quicker if exposed to air. With all the other factors (sun, rain, debris, etc.) I still chose to buy a cover. Look on Amazon, a lot of people feel there are aftermarket covers that work better than the Weber ones.

A thermometer takes out a lot of the guesswork and I'd highly recommend one; get yourself an instant read. Weber makes a great one for ten bucks: http://www.amazon.com/gp/aw/d/B009IH...kyL&ref=plSrch

Aside from the tools, I highly suggest spending some time reading up on grilling techniques. If terms like multi-zone cooking, direct vs. indirect heat, and reverse sear are not familiar, start reading. Figuring out your favorite grilling techniques will be far more important than choosing one brush over another.