Building a sub enclosure is no easy business! You have to match the internal air volume to the speaker. Some speakers are designed to work with large air volumes (like the ones in night clubs) these are generally the most powerful and have the greatest low frequency response, others are designed for smaller enclosures and some are designed for no enclosures (free-air subs). If you don't match the enclosure to the speaker you will either get a "coughing" speaker or too "boomy" bass, it's a tricky thing to get right. There are some guides around:
http://www.crutchfield.com/learn/learningcenter/car/subwoofers/box_building.html?page=2
Don't forget to put some sound deadening material in the enclosure to prevent standing waves, they sound sh*t.
I dont know much about your orion sub, but in my experience any sub for 16quid isnt going to blow you away, you were probably better sticking with the bose i'm afraid. Stick to a well known brand, Alpine make some sweet subs (Type R or Type X) which you can get pretty reasonably from eBay:
A personal favourite shop of mine:
http://stores.ebay.co.uk/Autosounds4less-Public-Warehouse
To choose an amp you need to know what impedance the speaker is (4 or 8 ohm usually) and the RMS power handling of the speaker. Then find an amp that has similar RMS(not MAX) power output at that impedance.
Good luck, let us know how you get on!