Having done this more than once (on building #4) ...
If you want to elevate it, concrete pier blocks are a good option, or you can set 4x4s in concrete and build up from there. (pictures below)
I added a BUNCH more wood to my foundation and floor - because it's inexpensive to do --- and almost impossible to do later! I went a little over the top with the 9" spacing for my floor frame but ... I'll never have to deal with a sagging plywood floor. Use 3/4" plywood for the floor. Again, once it's in - that's it!
Most of these kits don't come with a floor and I consider that a BLESSING- because it would just be the 1/2" particle board s**t.
Some friendly suggestions: DON'T USE THE HARDWARE IT COMES WITH. I used all screws everywgere - including attaching the siding because it's much easier to change things later (and right then if an error happens.
Star drive screws are MUCH harder to strip than philips or flat head screws. And trust me- they don't send high quality screws!!!! Home Depot has a line of them that ALL use the T25 drill bit, regardless of length. The links are to the basic product - they come in lots of sizes.
GRK Fasteners #9 x 2-1/2 in. Star Drive Bugle Head R4 Multi-Purpose Wood Screw (300-Pack)-100101 - The Home Depot
Instead of 3/8" lag screws, I found HeadLOK structural wood screws so very easy to deal with. They come in a variety of lengths and package sizing - also kind of a star design head or there's also a hex head version. SO SO easy to deal with. Even the 6" zip right on in.
FastenMaster 3/16 in. 2-7/8 in. HeadLok Star Drive, Flat Head Wood Screw (12- Pack)-FMHLGM278-12 - The Home Depot
I strongly recommend putting horse stall mats over your plywood floor deck. This does a couple things. First, it will preserve the life of the plywood floor and it will keep all the things that they do to that floor (spilling water, pooping, wet muddy feet) from sinking in. I know some use linoleum - but the stall mats give excellent traction and stand up to just about anything. They also make a nice gasket for the building. Of course you'll need longer screws when attaching the wall frames to the floor frames, but otherwise it's simple enough.
Adding a ridge vent makes a big difference when it comes to ventilation.
Using hopper windows is another of my favorite tricks because when open, the glass is totally horizontal, leaving the entire window opening available for ventilation - instead of 1/2 the space being glass (which will build more heat in summer).
TAFCO WINDOWS 31.75 in. x 15.75 in. Hopper Vinyl Window with Screen-PV-HOP 32x16 - The Home Depot
Another way to add to the ventilation is to replace the shed door that comes with a kit with a security screen door. It will be a little more work because of the double door situation. Personally I would exclude the double door - use a metal screen door- and then do a "side light" with hardware cloth frames on either side for additional ventilation. Or you can rig it so it's behind the solid door since you're in a colder climate --
Unique Home Designs 36 in. x 80 in. Su Casa Black Surface Mount Outswing Steel Security Door with Expanded Metal Screen-5SH202BLACK36 - The Home Depot
In mine, I installed a barn fan (made for dusty conditions, unlike anything that's for a house, like attic fans which state not for use in animal facilities) opposite my metal security door. It's just like framing a window- pop into the hole, 4 screws later - plug it in, and done.
Indoor/Outdoor Shutter Barn Fan J&D MFG - Stalls | Stable Equipment Supplies (valleyvet.com)
As others have mentioned, extending the roof overhang on all sides is a GREAT way to shade the walls and PROTECT your siding from weather. You'll have to get some additional plywood, but it's worth it. Again, really and truly- a ridge vent will help soooo much, especially in a cold climate where expelling moisture is vital to preventing frostbite. Also much harder to add later- so do it up front.
Photos:
Foundations:
Concrete encased 4x4s
Horse stall mats over the floor- they come in 4x6 and 4x8ft sizes:
This is my pier block version of the foundation. The kit shed behind it was for goats, so they didn't get the security screen door.