Hi Gavin,
The ASA5500 series has no concept like PIX where you can buy dedicated unit doing FO, and with cheaper price that the primary unit.
You need to buy same model, same product no. By default, if I am not wrong, it support Active/Active (A/A) mode. To have active/standby (A/S), you need to get Security Plus license. This allows you to always choose either A/A or A/S to run at any time (not both).
Refer to the url for ASA5510 actual model+features:
Table 10. Ordering Information
http://www.cisco.com/en/US/products/ps6120/products_data_sheet0900aecd802930c5.html
Rgds,
AK