I would like to write my own cryptocoin, but instead of attempting to write it totally from scratch (should I?) I was thinking it might be easier to modify a pre-existing blockchain. If so, which is the simplest or perhaps the easiest to digest, and modify into a custom coin? Thanks.
Firstly you should understand that software begets the blockchain. Secondly, you need to understand that you can't modify a pre-existing blockchain; by definition, the blockchain is a growing list of records (blocks) that are secured by a link to the previous block to prevent tampering. It is improbable that you will find a modifiable pre-existing blockchain - though I have no idea how much hashing power you have at your disposal.