Unfortunately, digital scanners are top of the line models so there isn't really an "entry level" digital scanner. By "portable" I assume you mean handheld so your 2 main options are the Uniden 396 or Radio Shack PRO-96. For a beginner, I would probably recommend the PRO-96 because it seems to perform a little better, and has less bells and whistles than the Uniden, so it is a little less complicated. Either way, you are lookng at about $500 for a new one. Check the for sale forum here on RR and also check out eBay, you can probably find a decent deal on a used scanner. There is also an older Uniden model...the 296 that would work for you as well. They don't make these anymore, so your only option is a used one.
Also, the Uniden 796 and 996 and the Radio Shack PRO-2096 would work for you. These are models that can be used either as a base unit or mounted in a vehicle.
Where are you? You may not need a digital scanner. Check the RR database for info on radio systems in your area.