If I were you I'd make sure the drive is the problem before assuming the drive is bad. MBPs from that era are also known to have problems with bad SATA cables as well. It might be worth your effort to read the drive problems article at http://scsc-online.com in their How To section. I'd do this first because it can tell you about a lot of the problems that are drive problems and others that can act just like drive problems. Cable problems and drive problems can act nearly identical. There are other problems that can exist that have nothing to do with the drive at all.
Since you're pressed for money, one way to at least isolate it to either the drive/cable would be to boot off anther external drive instead of using the internal drive. Once booted, used Disk Utility to completely unmount the internal drive and run off the external drive exclusively. This will take it out of the equation completely. If problems continue it's probably the system or some type of software conflict. If problems disappear, then you've at least got it down to the cable or drive.
If you determine it's the cable or drive, one trick I that may be useful is to print out the repair instructions from iFixit.com and when you remove the screws use masking tape to affix them to the instructions where they came out. Not all the screws are the same size/length and I've seen cases where people actually forced screws that weren't the right size into the wrong place and punch right through the logic board or other components. Also be very gentle removing any connections that plug into the logic board. It's a good idea to use a very small screwdriver to work them loose by putting the blade of the screwdriver between the junction of the two connectors and gently rotating the blade back and forth to work them loose. Sometimes on older units due to heat and age the connectors can oxidize and stick a little and just pulling them out may actually pull the connector on the logic board right off of it. Be gentle and be patient.
Hope this helps.
