I live 30 km from Germany and drive there quite frequently.... I'll give you a little summary. Hope it's useful for others as well.
1. Umweltplakete
Environmental
sticker, costs 5 Euro to obtain. You can get it from basically any random car dealership for 5 Euro. Doesn't have to be SEAT (or VW/Audi/Skoda). I got mine from a Mitsubishi dealer. You'll need your V5C though. Any
Leon Mk3 is entitled to a green certificate as all cars newer than (no idea - 2012 or so) get a green one. As mentioned, the fine for not having one is 80. *If* you take your chances without one - I recommend NOT to park in the street (where traffic wardens may check) - park it in a private garage where city traffic wardens are unlikely to check. They can and will issue fines for parked cars not having the
sticker!
2. Speed
2.1 Speed signs
Speed signs look the same as in the UK, but they're SMALL. Incredibly small, which is very annoying, as it's very easy to miss them (both the start of a limited section, and the end of it). Luckily the sat-nav has the speeds in it, but keep in mind it's not always accurate (sometimes a section gets changed). Note electronic signs are binding and overrule regular signs, as in the UK (M25, M1 etc).
2.1 Autobahn speed
Where there is no limit, people do indeed drive 200-250 kph (like, up to 150 mph or so). Personally I usually do between 170 and 200 kph (my max is 212 kph). So indeed: watch your mirrors. And keep in mind the danger. If you want to hit it on its tail, go ahead, but...
a) don't do it on a BUSY road
b) maintain proper distance at all times
c) you have to THINK AHEAD very far! especially on dual-lane motorway (ie, 2 lanes per direction, not 3) - look ahead for cars doing 120 kph approaching lorries doing 90 kph - you KNOW they are going to come left at any moment! you need to look *much* further ahead than you might be used to from the UK, as the differences in speed can be massive!
d) in the dark, when driving (very) fast, there is the risk of deer and other animals on the Autobahn sometimes - be warned, it's at your own risk
e) in the dark, you may want to consider turning on your front fogs for increased visibility - do at your own risk as you can get fined for using them when it's not severely foggy or extremely bad weather - I often find the safer feeling worth the risk of getting a fine (which is not that high);
f) anyone involved in an accident at above 130 kph may get a fine regardless of the cause; also, your insurer may give you trouble!
g) in work zones, be careful to maintain the (usually) 80 kph speed limit, and 60 kph when indicated; you can use cruise control - there are regular speed checks in work areas
h) when driving very fast, be very careful with sharp bends in the Autobahn - some are extremely treacherous! (example, Autobahn 66 between Wiesbaden and Frankfurt) - always slow down before sharp curves!
3. Sat-nav
As previously described, works very well. Recommend to have the speedometer in the small display though, not navigation. Have navigation in the main display.
Some VERY useful tips, I think:
* use the digital speedometer in your small display - I always have mine set to it; you can change it from mph to kph in the settings!
* for fuel, to keep the cost down, DON'T use Autobahn service stations - rather, look for the signs announcing "Autohof" which is a service station right off an Autobahn junction (like, literally, 100m off the Autobahn usually) - these are much cheaper (like 15ct per liter) than the ones directly on the Autobahn. (Autohof is like many (most) UK motorway services where you actually exit the motorway a bit)
) but previous RNS315 was ok and would stay in whichever country you set. 
