Yes, you should be VERY concerned about the leak. With that many miles and with an owner that wouldn't even bother to keep it clean underneath, the odds are HIGH that this engine is worn out.

Even if you do all the work yourself and be as economical as you can on the engine rebuild it will cost a minimum of about $2000 US. To have it done by an independent shop would certainly top $4,000 US.

Good luck,