I have just been told by my local mechanic that my camshaft oil seal has gone thus my 2001 2.0 Zetec Focus is leaking oil badly. He said he would do it for £250 cash.....Is it a job I could do myself (I am ok with tools....not the best though but can do a basic service, change brakes etc) and is it ok to drive with this thing leaking?

Thanks in advance