Welcome and nice work on the thread revival
This is just my opinion here, but if I were in your shoes, I'd be putting down a good ol' kit n kilo brew as your first, just to get a feel for the basics and ensure all your equipment is functioning correctly.
At the end of the day, it's just a recipe I suppose. So if you're confident your gear is complete and functional, and you're confident you can follow instructions accurately, then you can certainly have a go at this one straight out of the gate.
Two really important things here are sanitation and temperature control.
Make sure you have a good cleaner and sanitizer and use them according to directions. Don't be tempted to add an extra 10mls in order to make a stronger mix, as it will have no extra benefits.
As for temp control, lagers almost always need to be fermented at relatively low temps, then "lagered" for an extended time. ie. bottled/kegged then kept at 1-4 deg for a month or two. (In very general, sweeping terms)
For the two yeasts in this recipe, 20deg just ain't gonna cut it. They are proper lager yeasts and require lower temps to get the right flavour profile.
You could try using S189 Swiss Lager yeast at 19 deg if you are unable to hit the lower temps. Ross at Craftbrewer (sponsor above) uses it with great success to make his lagers @ 19deg.
I ferment and bottle in my shed, but it's a brick shed at the base of a 3 story unit. Concrete floor and ceiling w/3 brick walls and a steel door makes for average temps 4 or 5 degs lower than outside (during hotter weather). I ferment in a 95L esky which I stack a few ice packs in and that lowers the temp about 5deg lower than the shed temp. I have a Marzen that is sitting at around 12deg and has been there for a week now, just rotating the ice packs.
If you plan to use the shed, get your hands on a cheap indoor/outdoor thermometer and measure the temps for a few days to see what steps (if any) you need to take insulation-wise.
Hope you can make use of some of these tips...
