Yeah, you can certainly get a good PC for cheaper, without a doubt. £450 is pretty good, you spotted a good deal their man. But to an outsider, that's double a console, for no extra performance. In all honesty, the Xbox One graphics have been pretty great, so I can see why someone wouldn't spend double to get not much more.
If you're open to suggestions, I've went through a lot of mods and found some good ones. They may not be your style, but you might enjoy these -
Apps
Helicorsa. Aids seeing cars around you, for safer racing
http://www.racedepartment.com/downloads/helicorsa.5199/
Pitconfig. Allows you to select pit options before getting to the pits
http://www.racedepartment.com/downloads/pitconfig.8032/
Sidekick. Well laid out dashboard, with customisable options
http://www.racedepartment.com/downloads/sidekick.11007/
Setupmarket. Download setups in game
http://www.racedepartment.com/downlo...-market.10149/
Tracks
Donington Park. Very very nice build.
http://www.racedepartment.com/downlo...ton-park.3031/
Le Mans. Not great, but the best version currently
http://www.racedepartment.com/downlo...a-sarthe.2482/
Bridgehampton. Old American circuit. Amazing in 70s and 80s cars
http://www.racedepartment.com/downlo...-circuit.6604/
Paul Ricard. It's stripey.
http://www.racedepartment.com/downlo...l-ricard.6115/
Riverside. Same author as Bridgehamton. Very good build.
http://www.racedepartment.com/downlo...-raceway.9492/
Longford 1967. Updated GPL track, but updated a lot.
http://www.racedepartment.com/downlo...ord-1967.2741/
60s Hockenheim. It's fast.
http://www.racedepartment.com/downlo...-tracks.13105/
Misc
Better damage display
http://www.racedepartment.com/downlo...uc1gnol0.4736/
Skins/Skin Packs
Black Falcon AMG GT3 Collection
http://www.racedepartment.com/downlo...kin-pack.8471/
HTP AMG GT3 Collection
http://www.racedepartment.com/downlo...kin-pack.9870/
Nismo 370z mega pack - great if you want a one make series
http://www.racedepartment.com/downlo...ga-pack.10522/
917K Martini Pack
http://www.racedepartment.com/downlo...ni-pack.13165/
Porsche GT3 Supercup Skins
http://www.racedepartment.com/downlo...in-pack.13140/
Porsche 962C Pack
http://www.racedepartment.com/downlo...erypack.12781/
AAI Corvette
http://www.racedepartment.com/downlo...ns-2016.10945/
There's a ridiculous amount of mods, but those are the best tracks I've found so far.