Eventing is awesome!! Be aware though, it's addicting!!
I don't think you necessarily need to go out and buy a whole new saddle for eventing. I evented for years in just my close contact jumping saddle, all the way up to prelim, before I finally got a dressage saddle. That said, if you're in the market anyways, that saddle would probably work just fine. You'd have to sit in it though, I have a few friends who had HDR's in the past that put them in kind of a peculiar position, and ended up having to get a different saddle. They're not the greatest in terms of quality, I'd probably recommend going the used saddle route instead of buying new, you'd get more bang for your buck in terms of quality, but again, if it fits you and your horse and you're happy with it, that's all that matters!
The boots would definitely work, although I think they are fairly pricey as well. At the lower levels and just starting out, open fronts or regular splint boots are perfectly acceptable. Once you start getting more serious and moving up, then I'd invest in a good set of cross country boots. Woof boots, Dalmars, Nunn Finer, Premier Equine, there are so many different ones!
The figure eight bridle would be fine for eventing, pretty much any bridle would be just fine. You don't NEED a breastplate, if your saddle tends to slip back or you need to put a running martingale on it would be handy though. Depends on the horse. I would definitely go with elastic though if you decide to get one, much more comfortable and less restrictive for the horse.
As for vests, a certified vest isn't required, so you can shop around and find one you like. I just have the regular Tipperary one that isn't certified, it's light and comfortable. There are lots of options though! It being certified is a personal choice now since it isn't required, so it's up to you. Personally I don't feel the Tipps are any less protective that the others, but it's all about what you want.
One of the good thing about eventing, is that there's no "dress code" like there is in hunters or jumpers. You aren't going to get marked down or frowned upon for having the "wrong tack" or "wrong color", etc. Whatever tack that you currently have is usually more than sufficient, provided it's clean and in good repair, and is legal for eventing. I don't think a "shopping spree" is necessary at all, especially when someone is just starting out. But all the stuff you posted would be just fine for eventing