I'd stay with the 25mm tyres, they have less rolling resistance (but more air drag), though I don't want to start some sort of holy war, tyres are an almost religious question ;-). The spec'd Michelin Pro² are quite nice.
If your average speed exceeds 30km/h, air drag starts to be the dominating effect, though your position will have a greater influence than the tyres (and of course, the wheels), the aero bars will amend that. If you flip over the stem, you most probably also have to move the saddle more to the front (I ride a +0 mm seat post with the saddle as far to the front as possible, but mine is a road bike), as your hip angle will decrease quite a bit. Experiment with handle bar and saddle position. A low position does not help if you get tired or cannot stay in that low position for the complete ride!
I'd stay with the pedals and shoes you have at the moment. Triathletes wear special tri bike shoes, where the velcro fasteners open to the _outside_. They leave the shoes on the bike, run barefoot with the bike through T1 to the bike course, jump on and close the shoes while riding... this is not what you want to do from the beginning on (and many age groupers don't do this).
Also, special tri shoes are expensive. I ride road shoes (I couldn't find good fitting tri shoes), where the fasteners open to the _inside_ and they start clapping against the spokes... I'd not recommend that to anybody (if I crash, it is my bloody own fault!). Ride your first triathlons with the (bike?) shoes you have and don't hurry too much during the transitions. They are for beginners the most difficult part (organisationally and physically): After the swim, your legs will have to get accustomed to supporting your weight again, after the bike, your legs will want to continue going round and round. If you hurry too much, you will probably get cramps.
Also: practice the transitions, this is called a "brick workout": bike about 10-15km and do a short run afterwards (3km). To speed up the transitions, use elastic bands instead of regular shoe laces in your running shoes. Bike and run in your swim shorts or spend the 50 on a tri short (they have thin padding for biking and dry very fast), if you have the money and know that you will continue with this great sport. If you have beginner's questions concerning triathlons, ask me, look in the tri forums or search pages like http://www.slowtwitch.com
. I am still quite new to the sport (this is my 3rd year), but I probably still remember the problems I had (and still sometimes have).
Sorry, this is a bit longer than I planned, but I got carried away...