I'm no car guru, so i'll advise the best as i can mate.
2006 I've got a 2.0R legacy and completely love it. Really nice to drive and the AWD is great in the winter and gives the car a nice solid/planted feeling when driving.
If i was to get another legacy, I'd almost certainly get a 3.0ltr rather than a 2.0 or a 2.5ltr. The 3ltr is a 6clyinder engine and yes it is chain driven. Both the 2ltr and 2.5ltr's are not chain driven. I believe the 3ltrs are about 240bhp compared to about 165ish in the 2.0R's. Certainly worth checking out the service history. Oh and the 3ltr post 2006 versions are £480 per year in tax. The pre 2006 versions are £285 afaik.
I cant comment on the mpg of the 3ltr's but it will run best on shell vpower or tesco momentum. in my 2.0 I tend to get around 35mpg the majority of the time.
With the legacy's you have to watch the rear wheel arches(there's a lip that tends to collect **** and rusts). Also if the exhaust is a twin exit, you will need to check the Y piece, as these tend to rot/rust - easiest solution is to have a stainless steel exhaust fitted cost ya about 400-500 quid. - Unless its been done already that is.
oh and I find legacy parts easily available, importcarparts and Ebay tend to sell everything required. There's a seller on ebay called Impartsb46 who sells genuine subaru parts brand new and cheap.
Hope this helps. We have a couple of 3ltr members that I'm sure will be able to advise :)