It is true that 100Hz can exaserbate(SP?) MPEG digital artefacts. You notice them on digital TV (until you get used to it) where the encoding is higher but you should only see it on badly encoded DVDs like 'Lost in Space' R1 or 'Dune'R1, even then it should be only mild in the background of some scenes. Certainly DVD's like 'The Matrix' or 'Trainspotting' should have no noticable artefacts or dithering at all.
Try swapping from one Scart port to another, switching the type of output, ie RGB, S-vid, composite, and even try the S-vid (4-pin mini-din) output on the back of the DVD player. Basically, try and eliminate and re-create the problem by changing as many things as you can. If you can't then perhaps something is more seriously wrong.
Do you have a digibox? Do you see similar problems with that? FYI Digtal TV uses the same compression technique as DVD so if your digibox is fine using the same scart port as the DVD and same interconnect signal type e.g. RGB then your DVD player is probably a duffer.
Failing that, try a diferent player, then try a different TV, both 100Hz and 50Hz.
In conclusion, your dealer is talking out of his hat. The picture should be fantastic almost all of the time and should kick the Sh*t out of your VCR no problem.