For good measure, I would suggest replacing Front and rear diff as well as the transfer case. You will already be under the truck with the tools out, so what is an extra 30-45 min of time to have fresh fluid in everything?
You should be fine. I have a Snowdogg VMD75 V-Plow on my Silverado 1500 LTZ. I also have a leveling kit to eliminate sag, but do not keep the plow on all the time. At this point I just have the plow on as a back up, but it gets the job done. I believe you will be fine with a straight blade.
It is either a D2S or a D4S bulb
You can tell the difference between D2 and D4 lights looking at the bulb base.
But you can probably determine what you need from this link here:
https://kensun.com/blogs/news/factory-hid-bulb-types
The installation took me about 2-2.5 hours and I did it by myself in my driveway. If you have a friend or an extra set of hands help you on the part where you hold the step up to the brackets and bolt them down that will make things easier. One other thing I will suggest is to use some blue loctite on the bracket bolts that hold the brackets to the truck. Mine vibrated loose and I found out it was a semi common thing with some trucks so I applied the Loctite (threadlocker) and it's been perfect since (over a year now). Other than that, just take your time and you will love them.
You can purchase a replacement gasket on Amazon, which is a lot thicker than the original was and it will fix your issue. I've gone through the same when I replaced/swapped out my bulbs a few times.