You basically have two choices. One is to use an infant seat for the first 6-12 months and then a rear facing seat (preferably to age four or longer). Second option is to purchase a rear facing seat which works from birth to around age four.
There are plenty of infant seats which work fine. With or without Isofix. Maxi Cosi Cabriofix is the most common option. After the first 6-12 months switch to a rear facing seat. Compact choices with good angle are Britax Hi-Way, Britax Max-Way and Axkid Minikid. These should all be used with a wedge for better angle. RF weight limit is 25 kg. There are some other seats which will also work but it will be more cramped in front seat.
HW, MW and MK are installed with seat belt. If you prefer Isofix then Kiss 2, Triofix Recline or Beafe X1 are the best choices. Good angle, fast installation but also the lower 18 kg rear facing weight limit.
If you prefer a rear facing seat from birth options are Klippan Kiss 2, Britax Max-Fix 2, Britax Dualfix or Concord Reverso. MaxFix and Dualfix are not great choices in your car due to the angle. Kiss 2 and Reverso will work fine. Kiss 2 has better angle and is a bit more compact. It can be installed with Isofix or seat belt.
Keep in mind that back seat in a Focus isn't spacious and also sloping more than most other cars. That means many seats will be quite upright. This is not dangerous but head of child falling forward when sleeping can be a little uncomfortable.
Isofix and seat belt installation provide the same safety. Isofix seats are faster and easier to install but are also more expensive, very heavy and have the lower 18 kg weight limit. Belted seats, at least the Swedish ones, have the higher 25 kg weigh limit. Belted seats are also far lighter and in most cases much cheaper.
Top tether is not really used in Europe. We use Isofix and a support leg since it's a far better solution.