Welcome.
I used to to shoot with a Sony Alpha 100, it was my first DSLR. I then traded up for the Sony Alpha 350 which was much better, particularly with live view, and auto focus.
Personally I had the Minolta 70-210mm f4 "Beercan", which was pretty good for what it was and only costs about £60. But not very long.
I also had the Minolta 50mm f1.7 which was a superb lens, better than the Sony version I had before that.
There are plenty of lens options for Sony A mount, you just have to know what to look for.
I suggest trawl the lens database on dyxum.com this has a complete list of every lens that was ever made for A mount cameras. Then use the search options (on the right hand side of the page) to search for any lens between 70 and 500 mm, this will give you all the long zoom lenses. Here's the search,
click here
Then find a lens, for example the Sigma 120-400mm, then search that on ebay or LCE (London Camera Exchange).
Not all of the camera shops sell old A-mount lenses. These ones do:
London Camera Exchange - for example, here's a search of all their Minolta (A-mount) lenses:
click
Camera World - listings of Minolta/Sony A-mount lenses:
click
Park Cameras -
listings of Sony A and E mount lenses here, be sure to check that it's A-mount if you order.
This is probably one of the best you can find of more modern A-mount lenses, the
Sony 70-300 G SSM for £369 but it's only a 70-300, which isn't that long for wildlife.
The advantage of the Sony Alpha 100 is that it has built in stabilisation, so even if you put on an old Minolta-era lens, it'll still get the benefit of the "Super Steady Shot".
The disadvantage is that it's a really old sensor and doesn't do well at high ISO in dark situations or when you need a faster shutter speed for wildlife.
On the plus side again, you can find some real bargains around of Sony A-mount cameras and lenses now that Sony have moved over to E-mount.
Happy Hunting