... and here I am in person!!!
First, you say your AVI clips are large, but you don't give an indicative size of one of the files. With the length given in seconds, knowing the size of the file would allow us to work out exactly what type of AVI it is. However, that being said, I suspect it is indeed either a raw, uncompressed AVI (which are huge, at about 65 GB per hour of video) or, more likely, a DV/AVI which is also large at around 13 GB per hour of video.
In other words, don't worry about the size at this end of the AVI scale. And if indeed it is DV/AVI, then quality-wise that is possibly the best in the standard definition world.
I note also it is Frame Based. But from my comments above, you will see that this is fine for a slideshow (at least one without video clips inserted).
However, I see that the mpeg properties you give contain a couple of anomalies. The first is the frame size (480 x 576). Can you confirm that again, please. Unfortunately, it is not one of the frame sizes which conforms to the international PAL DVD standard. However, it is the standard PAL frame size for an SVCD (a better quality version of the old VCD which is burned to a CD rather than a DVD...)
I am not sure what to suggest about that. If you convert that to DVD-compatible mpeg-2 frame size for PAL (720 x 576), the program would have to invent 240 extra pixels out of thin air to pad out the width. That would mean loss of quality. I guess, though, that you could always convert down to half-frame size (360 x 283) and probably retain better quality.
However, I note that you also say that the mpegs are Frame Based. As also noted above, that is fine if you only play those mpegs on your computer monitor or on a progressive scan digital TV. However, otherwise, they will need to be converted to either Upper or Lower Field First.
But the good news is that if you are not planning to use video from any other source, then it doesn't matter whether you choose Upper or Lower Field First for your full project. You only need to maintain the chosen field order throughout. Or if you use other video sourced from elsewhere, then first ascertain what Field Order it uses, and then use that Field Order for the entire project.