What should i expect?

I would expect you to be telling your dad's coworker the parts you would like in it. It all depends on what you use your computer for.

does this include monitor/keyboard/mouse/OS/case?

It's hard to beat this

http://www.tomshardware.com/reviews/budget-gaming-pc-ph...

Of course, check UK shops to see what those parts would cost in pounds rather than dollars. If you also need an OS though it's going to be a little tougher and you may need to drop the Video card to a 5770 and the CPU to an Athlon II X3 to allow for an OEM copy of Win 7

According to google £500 = $800 USD so I threw this together on newegg. This was done pretty quickly and I really didn't shop around for prices so I'm sure there is room for improvement. I hope this gives you a decent starting point.

i5-2500k - $219.99
http://www.newegg.com/Product/Product.aspx?Item=N82E168...

COOLER MASTER Hyper 212 Plus - - $49.99
http://www.newegg.com/Product/Product.aspx?Item=N82E168...

ASRocks Z68 Pro3 MOBO - - $119.99
http://www.newegg.com/Product/Product.aspx?Item=N82E168...

GSkill Ripjaws DDR3 1600 (2x2GB) - $45.99
http://www.newegg.com/Product/Product.aspx?Item=N82E168...

CORSAIR Enthusiast Series 650W - $89.99
http://www.newegg.com/Product/Product.aspx?Item=N82E168...

MSI Radeon HD 5770 - $139.99
http://www.newegg.com/Product/Product.aspx?Item=N82E168...

SAMSUNG Spinpoint F3 1TB HD 7200 RPM - $59.99
http://www.newegg.com/Product/Product.aspx?Item=N82E168...

COOLER MASTER HAF 912 - $59.99
http://www.newegg.com/Product/Product.aspx?Item=N82E168...

ASUS 24X DVD Burner - $20.99
http://www.newegg.com/Product/Product.aspx?Item=N82E168...

Total: $806.91


Again, I didn't spend a lot of time on this and there's probably a much better budget build out there but I hope it gets you started. If you're not interested in OCing you could easily drop the heatsink and perhaps go to a less expensive CPU or AMD chip. The money you save there could also get you a nicer GPU. It really depends on what you're wanting to do

-Steve

Thanks  

http://www.ebuyer.com/product/174597
http://www.aria.co.uk/SuperSpecials/Other+products/AMD+...
Select 4Gb of 1333 ram from these
http://www.aria.co.uk/Products/Components/Memory/

power with
http://www.aria.co.uk/Products/Components/Power+Supplie...

and shinies from
http://www.scan.co.uk/products/1gb-sapphire-hd-5770-pci...(x16)-4800mhz-gddr5-gpu-850mhz-800-cores-dl-dvi-i-hdmi-dp
allowing £40 for your ram that makes £305.67 (plus any postage ofc)
Case and O/s are ofc up to you but enjoy the change  
Moto