2006 nissan altima

I have a 2006 nissan altima with over 200,000
miles on it. My car has been really hard to start
lately, I have to keep trying to crank it over keeping
my hand continuously on the key  in order for it to
even try to crank over. And when it does finally
start the rpm will fluctuate all out of wack, and a lot
of white smoke comes out the tail pipe. I know the
radiator has a crack in it, and needs to be replaced
and I was told my head gasket might b either
cracked or blown, I bought the head gasket kit and
a new radiator to be installed but my question is
should I just save up for a new engine, cuz I have
been told that when you replace that kind of stuff
the new parts put more stress on the old parts and
I will have to replace anyways. Or should I just
replace the parts that I bought already, and see
what happens?