Knowing you are in the USA, and possibly switching countries helps a lot. Batteries don't care what country you are in, and chargers are not that expensive. Just another thought, although you might not want to pack everything back to Europe.
Festool has two types of battery powered tracksaws with one being the HKC with a non retractable blade, and the TSC55 which retracts the blade. The HKC is the saw for framing and rough carpentry. Although it will work on a longer rail it is really the framing saw. The TS series saws are the fine carpentry saws. Both cut wood!
Dewalt and Makita also have cordless tracksaws. Milwaukee does not have a tracksaw yet. Bosch is about to release a new battery tracksaw that uses the superior Mafell rails for a whole lot of money. (The Bosch rails connect together without a straight edge and the edge strip is captured so no double stick tape to fail.) I have all Festool myself and it works excellently as a system. Makita rails are known to have issues, so it is wise to check them for straightness.
On the rail length, you can attach two 55" (1400mm) rails together. To rip an 8 foot sheet down you will need two rails. With a circular saw you can use something like a piece of steel box tubing, or an extrusion like a level. (I own a long rail, but have two 55" rails that can be connected together. A level is what I use to make sure the rails are straight.)
If you are buying cordless battery powered tools always buy "brushless" if possible. The power difference is huge, and longevity of the tool is better. Makita, Milwaukee, Dewalt are what I call the big three, but people like to throw Bosch in the mix too. An impact gun is invaluable around a house. Drills just don't drive larger framing sized screws as well. Sometimes it is cheaper to buy a kit to get the batteries and charger. Other times it is cheaper to buy a bare tool that includes an extra battery, then buy the charger from Ebay. Home Depot often has decent deals on kits as do most of the bigger hardware stores. I bought a Makita saw for the 4 batteries, and still haven't used the saw.
Circular saws have a lot of options. Left and right hand blade. Being mostly right handed I prefer a left hand blade for circular saws. I have both left and right blades and use them both for different jobs. There are 6 1/2" blades and 7 1/4" blades. My preference is for the smaller 6 1/2" saw as it is easier to wield and throw around. There are times a 7 1/4" saw is required, but not for building a simple shed/tiny house. Cordless is probably for the best since you already have batteries. Hmmm...Ryobi what Ah are the batteries? Well if you have Dewalt batteries the tools tend to be a little bit better quality. Which batteries do you have and what is the Ah of them? Preferably over 4.0Ah.
https://www.dewalt.com/products/pow...%3b%3bCordless-s-%3b%3bCordless-s-|&pageNum=1
Not sure if you know about Restore? It is a second hand store (Lots of stuff is brand new) that is part of Habitat For Humanity. They are a non profit that builds homes for people in need. Anyhow the stores have doors, windows, and other building materials that tend to be very reasonably priced. They are not in every city though.
https://www.habitat.org/restores
Seriously watch a few "shed building" videos. There are far more tools needed, but most are pretty minor things like chalk lines, etc. You will consistently see folks using nail guns, screwing things with impacts, hammering nails in the old fashioned way, and about everything in between. Universally you will see a circular saw used in every build. An HKC is perfect for shed building, but by no means required. The HKC also doesn't come with batteries nor the short FSK rail, so cheap it is not.
https://www.festoolusa.com/products...saws/201359---hkc-55-li-eb-basic-usa#Overview
Chainsaws are super dangerous. Left leg and left hand are the most common injuries. Circular saws are no where even close. Say if you wanted to cut a 2x4 you clamp it down onto a couple of saw horses, which leaves both hands free to work the speed square, and the circular saw. Of course as you know most folks just stand on the 2x4, or hand hold it while cutting. If this is a two person project, one person could hold the 2x4 down while the second person cuts it.
https://www.homedepot.com/p/Husky-28-in-Folding-Sawhorse-2-Pack-206138/305264222
These are okay for cross cuts up to 50".
https://www.harborfreight.com/50-inch-clamp-and-cut-edge-guide-66581.html
Speed square use, one of hundreds on youtube.
Edit: This guy has a few videos on building sheds that seem pretty easy to understand. There are so many different ways to build a shed. You can find plans online. I would assume this is more of a play house? Not something with a bathroom, electricity, and...?
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