Chosen Solution
Besides the troubleshooting you already attempted I’d try to make sure the hard drive is in working order. Depending on the Os version you have installed you can try getting into recovery partition or try with the install disk and see if the boot drive works properly with disk utility. Getting a replacement board depends very much on your future expectations and needs..these Macs are surely dated but they still work nicely for most leisure needs and they are relatively easy and inexpensive to upgrade too.
