For removing the tar, get a scraper or and old chisel. My was quite hard & brittle and chipped off where is was thick. Some people recommend a heat gun, but I reckon cold and brittle makes it easier. Then Wire wheel in an angle grinder.
I rolled my mini over onto it's own wheel, but if doing it all again would build a rotisserie.
The shell it quite light. I'm only a little bloke but can roll it around all day with ease. When pushed, can also pick the lot up with the help of who ever is around (including the GF).
Here you can see it sitting on the tyres:
If you are going to take the time to build a trolley, spend the little extra to make it a rotisserie.
And don't skimp on the wheels. The $2 jobbies from Supercrap are made of this lame rubbery stuff that eventually comes appart.