You know what you can afford, you know what tools you have and what skills you have, base your decision on these things.
After the purchase will I be able and can I make this what I need it to be as soon as I want it to be.....?
If it's a good runner as it is and you like the 49cc 4 smoke set up I say buy the heck outta it.
Its only a good deal if you can afford it and you are able to fix and/or build it up into what will be suitable for your wants and needs.
That seems like a good price to me if it has a good engine, from what I've seen just the bike alone would cost you close to that.
Best wishes, map