Hey! Welcome to the community
Firstly, very smart move, switching to propresenter. Most music videos you buy in iTunes are m4v, which is basically mp4, but with copy protection. But ya, propresenter can play them. Generally as a rule of thumb with propresenter, if your machine can play the video, propresenter can too. Are you going to be running it on a Mac or PC?
Setting automatic timers is pretty easy. You can either right click each slide and select the "go to next timer" option, and set how long you want a slide to show, then on the last slide right click and choose "loop to first slide". That will make it start the slides over when the last one finishes.
That is the method I use because I like to have different times for some slides, but if you want every slide to be the same length, there's also an icon on the top right that looks like a little stopwatch, if you click that and choose a time, it'll automatically set all the slides to that length and add the loop to first slide option to the last slide.
The countdown is a little more complicated to set up, but once you do, it's super simple to maintain and use. First you want to create a template for it. Mine is just plain text kind of small all the way at the bottom of the screen over a slightly opaque black rectangle. To make a template, you need to design a slide to look the way you want it to, command click the slide to select it, then click the template button on the top left and choose "save selected slide as template".
Next you need to click on the "clocks" button on the top in the middle and set one of the countdowns to the time that your service starts. The "countdown to time" is nice because you can start it whenever.
Next you need to go to "messages" also on the top in the middle, and create a new message. You need to type your message in the box. For instance mine says "Service will begin in" then you drag the token for whatever countdown you used into the box after the text you typed. So my finished message says "Service will begin in (Countdown 1)". Then choose your template that you created earlier, and click "show" and voila! Your countdown will display independently of the announcement slides.
Hope this helped. It's kinda a lot of info to squeeze into one message, so let me know if I need to expand on anything. I'm always happy to help!
Nate