DIYbanter - conflicting advice on roof repair

I have a 400 square foot roof that is leaking. It is an almost flat
roof next to a more slanted roof over my garage. I got an estimate from
a roofer for $2800 who said the roof was shot. He said he wants to rip
off the roof and the roof of the garage. There is nothing wrong with
the garage roof but I guess the junction between the two roofs could be
the source of leaks. I thought I'd save some money and do the work
myself and just lay shingles over the existing roof. I went to Home
Depot and was told that putting new shingles over old shingles was a
bad idea because the old shingles would swell and crack and damage my
new shingles. So I decided to rip off the roof myself. I started
writing down UPC numbers of tar paper and shingle when a roofer came
along and told me that I was wasting my money by doing it myself. He
told me that roofing is an intricate and delicate business. He said
that you need equipment that heats up the tar. He said cold tar won't
last. I was just planning to spread tar that Home Depot had near the
shingles. He said he would just lay shingle over the shingles that were
already there for $1700.00. I told him that two people had told me that
the shingles underneath will swell up and break the new shingle. He
said he has put shingle over other shingle before and hasn't had any
problems. He said if I wanted him to rip up the shingle that was
already there he would do it for an extra $500.00 so the total would be
$2200.00.
What should I do? Should I repair the roof myself or not? If I do, do I
need hot tar or can I get away with cold tar, if I do need hot tar do I
need to rent special equipment to heat the tar? Is it OK to just lay
shingle over preexisting shingle or should the preexisting shingle be
ripped up? I've also noticed that there are coatings for roofs. Should
I just put one of those on instead of new shingles?