I would imagine it is self install but who knows if the engineer can be sweet-talked into setting it up.
Any B/G/N adapters should be compatible with the router but you will get the best speeds using a D-Link N adapter. However, as you only have 20Mb/s going out the modem, spending extra to get 150Mb/s between laptop and router seems a little pointless (unless you want to transfer large amounts of data around the home - eg streaming movies from desktop to laptop).
My favourite supplier,
Novatech, has Edimax and own brand N USB adapters from £12 upto £35.