Ok first of all a 2007 MBP is worth a lot more than $300 and putting an SSD in the thing should give a high percentage return on whatever you spend for the SSD.  Secondly I just went through a round of SSD upgrades in all our old Macs.  I gave everybody small SSD drives for the OS and turned their hard drives into external drives with $10 SATA to USB enclosures.  It was the single most significant speed upgrade I've ever seen and I've been upgrading computers a long time.
On my 2008 Penryn (turned in my Santa Rosa for it), I installed a Seagate Momentus XT 500 GB "hybrid" SSD drive which is a 500 GB 7200 drive with a 4GB SSD drive.  All the decisions about what goes to SSD and what goes to HDD are handled by the firmware using the same algorithm that is used for its 32MB cache.  Everything is always written to HDD but frequently used content is prefetched to SSD.  It works great. It's not as fast as SSD but it's a heck of a lot faster than 5400 RPM and maybe twice as fast as a normal 7200 RPM.  For just over $100 that's a good deal and perhaps something you should consider. While you've got the thing opened up, don't forget to pop in more RAM.  
As far as the longevity of your hardware?  I expect it to go another 3+ years if you can stand the temptation not to upgrade when some fancy new 3d bluray  3 terrabyte 12 gigahertz for $99 Mac Plus (or whatver it's called) comes out.  
