1) You are correct, one should not use "when I'll manage to" in this construction. Remember that "I'll" is a contraction of "I will" (Or, rarely, of "I shall") and "will" here would be unneeded.
2) if the 2nd clause were independent and could stand on its own, a comma would be normal, but it is not. If you considered "when I manage to afford its price" to be a parenthetical clause, it could be set off by commas, that is have a comma before AND after. However, this is not clearly parenthetical, so the commas could well be omitted. But both or neither should be used.
In addition, one should say either "after a few months" or "a few months from now". The indefinite article "a" is needed in this construction, and one should not use both "after" and "from now", that is redundant and feels awkward.
The sentence could be recast as:
So I went to Blala market, and I saw a nice blue Ford, and I hope that it won't be already sold, when I manage to afford its price, a few months from now.
However, this feels run-on, and could be significantly improved. For example:
So I went to Blala market, where I saw a nice blue Ford. I hope it won't have been sold a few months from now, when I will be able to afford it.