Newbie looking for a new bike and not sure what to get?

There's always another bike on eBay or Gumtree...

You mentioned gearing for hills - to see much difference over what you've got currently you should be looking at a compact front (50/34) and an 11-32 at the rear. If you're looking at older bikes Ultegra 6700 had the option of an 11-30 cassette.

All the Shimano current gen groups are excellent - Tiagra 4700 is the sweet spot, just upgrade the brake calipers to 105 (£40, about an hour's work). With the long cage derailleur you can even run an 11-34.

Tarmac is more aggressive than allez which is more aggressive than  the defy. All frames are good but the tarmac is carbon and has the best group set. Be aware that giant l I think is bigger than spec 56 so I don’t think these are all equivalent sizes. Would be good to sit on a bike before buying

The Allez and the Giant have compact cranks. If you're just starting out, they should get you up the hills more easily than the 52/39 on the Tarmac.

The Allez and the Giant have a very upright position. The Tarmac is more stretched out.

All depends on what you like doing.

As to buying, make an offer about 10% less than the asking price. Most sellers will accept straight off.