DIYer Flue Questions

[MartyP]

Hi,
I am replacing my oil furnace (dead) with an LP furnace. I am also replacing the water tank with on-demand hot water.
Questions:
Can I use my existing chimney with a stainless liner to vent either (*) of these *
Is there a way to combine the flues so that there is only one exit hole in the *
Do you have any tips for saving on the stainless steel vent for the hot water ($200 in parts...ouch).

Thanks,
Marty