If these things are enough to obscure a file very much depends on who you are obscuring it for. As indicated in the previous question, there are still quite a few people that have ample knowledge on bits and bytes. If you are targeting not-so-tech-savy people then basically anything would suffice (as long as it doesn't show much in the default application or text editor you'd be safe).
The seed in this case can be seen as a key for an algorithm to create a key stream. Now you assume a "good" pseudo random number generator (PRNG). This is a cryptography form, so we would translate that into a Cryptographically Secure PRNG or CSPRNG. If the PRNG isn't a CSPRNG we can retrieve information about the state and reverse the process. If the seed is too small (less than, say, 64 bit or smaller) then we might be able to brute force it given enough CPU time.
Finally, if you ever repeat a key stream you are lost. Chances are that very high that files contain known plaintext (e.g. a JPEG header). In that case the key stream is leaked and any other information can be directly decrypted. Otherwise frequency analysis and crib dragging will quickly reveal the key stream with the same result. Any crypto student will have such a program available to them (it's often the first programming assignment).
Basically you are inventing your own stream cipher here. Using a well known one (AES-CTR with a random IV) is probably easier than all the trouble you are going through. Or simply use PGP for file encryption.