----- If you're forming ABS then you'll need a regular vacuum pump such as those used by HVAC technicians.
----- The sheet will always stretch but generally sharp outside corners or deep holes are what cause breaks. It may take some creativity to get to roughly 21" (50cm) high but it should be possible.
---- I form fender flares for off-road Jeeps out of 3/16" ABS. I use a modified ProtoForm vac former. Plans are found here: Workshop Publishing
There are some very good build threads for the ProtoForm vac formers on TK560. Look for the Discussion link towards the bottom of the left hand navigation bar. I use the same screen name there.