Get yourself a bitrate calculator... look at the overall length of video that you are using, and type that into the tool, then set the kind of audio you are using and add that in too - the calculator will give you a ballpark figure which will get the footage onto the disc. However, it *wont* necessarily give you a nice looking result, as encoding is way too much like an art for that to happen!
Google 'bitrate calculator' and you'll find plenty.
You can go way over 4.5Mbps, depending on the length of the material you are using. The maximum for video bit rate is 9.8Mbps, however there is very little point going this high unless you know why you must. For replicated discs (i.e. those you send to a bureau to be made) you can go to 9.8. For anything that you make in your DVD burner, or a duplicator, keep the video bitrate down to no more than 7.5mbps or so, and make sure you use AC3 (Dolby Digital) audio, which has a far lower bitrate than PCM (or .aiff) audio.
Menu graphics, subtitles, ROM content and so on all have an impact, but it is tiny compared to the audio and video content. Remember - any menu graphics will be encoded to MPEG2 when the disc is built, so it doesn't matter what they were in Photoshop, they won't be that on the final product.