To answer the orginal question, is it worth cleaning up a tank or just buy a
new one?

I have done it, and it saves you some money.  I pulled the tank(the bolts
came of easily but I have no undercarriage rust).  I emptied the tank and
took off the hardware w/o destroying it (which could easily happen).  I
brought it to a sandblaster and for $40 he was going to clean it up.  But he
ruined that tank and I bought a less rusty one and started over.  I
repainted it and reinstalled and it looks great.  It was kind of a hassle to
be honest.  The fill valve is still quirky, and it doesn't have the
"reguired" 10 pressure test stamps, but no one has ever checked when filling
it.  They do sometimes have trouble filling it, and now I just wait til it's
empty because it fills easier when empty.

I would get a new one next time, but then I am not afraid to spend money on
my van.