the crankpulley oil seal sits inside the lower timing cover, costs about £10 ish
I would advise to replace it as they can leak.
the new chain has a tensioner, if you follow the haynes manual it will show you how to release the tension, I personally release the tensioner and then fit it, it's quite easy to do.
once the new chain has been installed it's best to crank over the engine via the crank pulley bolt just to make sure all is ok.
you should disconnect the battery whilst working on the engine, so it will get reset anyway 🙂