The 'SD' box that you've been offered is a 'Standard Definition' satellite digibox. I assume that the guy is offering to install (subscription free) satellite TV for �150. If so, you need to check out exactly what he's offering you:
He could be offering to provide a Freesat box, together with the dish, cabling and associated labour. (The boxes cost around �50, so that total is probably not unreasonable). The Freesat service (which is a joint venture between the BBC and ITV) provides more channels than Freeview. They're listed here:
http://www.freesat.co.uk/index.php?page=whatso n.Main
Alternatively he could be offering to provide a basic 'free to air' satellite digibox (again, together with the dish, cabling and labour). Such a box will receive all of the channels shown in black type on this list:
http://www.vowles-home.demon.co.uk/Sat/SkyFTA. htm
(NB: The 'Five' group of channels, including 'Five US and 'Fiver' can't be received by a basic 'free to air' satellite box).
Lastly he might be offering to provide a 'Freesat from Sky' box (with its associated viewing card, together with the dish, cabling and labour). 'Freesat from Sky' is a different service to 'Freesat', which Sky seems to have discontinued offering to new customers (but a lot of aerial installers and TV dealers will probably still have the boxes and viewing cards). A 'Freesat from Sky' box will get all of the channels which a basic 'free to air' box receives but, with the viewing card inserted, it will also get the 'Five' group.
If you want the extra channels, go for the SD box. Otherwise simply upgrading your aerial (so that you can get the Freeview channels again) would almost certainly be a cheaper option.
Ch