Value for money, I recommend looking at the JVC range. The main feature you need is 'steering remote control ready', meaning it will have a socket at the back to accept a steering wheel control interface (eg: Connects2 CTSAR001 + CTJVC2LEAD).
To keep costs down, the audio signal to the BOSE amp can be via the speaker outputs on the new headunit. Audio quality isn't the best; the other method is to use the RCA preout terminals. This bypasses the internal amplifier, so the audio signal is higher quality. However, the 'preout voltage' needs to be relatively high; high enough to drive the BOSE amp loud enough. So you need a headunit spec'd with at least '4V preouts'; this puts you into the mid-spec and higher level of headunit = more $$. Made easier with a little adaptor, like the Connects2 PC2-36-AMPR.
Presuming you're changing from a 2-DIN size factory headunit to a 1-DIN version, you'll need something like a storage pocket to fill the lower hole. Connects2 offers the CT24AR06 fascia which is the surround + pocket.
Secondhand and ebay will also keep costs down, but you won't have good aftersales service and dubious warranty.