A lot of investors who are looking for smaller "renovations",  "wholetailing" is the gateway for them.

There are a number of towns (arguably most if not all of the Gateway Cities) where the numbers still work... for now at least. 

I then worked with both realtors on property search, but the second realtor I spoke with ended up just setting up an Auto MLS gateway and then never checked in, and was not particularly responsive   In our first call she referenced an investment property about to come on the market, and after I expressed interest she never followed back up.  

If you want to go off the deep end on risk, Fairview, North Camden, Waterfront South & Central, Bergen Square, Morgan Village, Whitman Park, Centerville and Gateway are some of the choices for deep discount/high risk/high maintenance - fix and flip or buy and holds.On bank financing, your going to run into some trouble. 

Sure there are plenty of low-income areas that are pretty rough around the edges: Federal Heights, Commerce City, Athmar Park, Westwood, Montbello, Gateway, parts of Thornton etc... the list of not-very-nice areas is actually quite long but there's really nowhere I wouldn't feel safe going into like you find in most other big cities, except maybe LoDo at 2am when all the bars are closing down and the streets fill up with drunken bros, or the parking lot of Mile High Stadium after the Broncos lose to the Chargers. 

I would consider comparing property radar with rebo gateway although if I understand correctly pr will give you a type of heat map that shows equity which is obviously huge especially when mailing to the high equity nod's like Rick H. loves to do.  

It's really about your starting funds you have and your goals.I do commercial so my clients tend to have 1 million, 5 10,15, 20 etc.The goal at that level is buy quality assets in gateway cities at good caps with small multiple upsides.

I have a signed contract with several people to be their gateway to free training with a national instructor, as well as a gateway to pre-approved lending on flips.

A rather interesting gateway book for this niche (if you are thinking smaller multifamily) is the aptly titled Investing in Duplexes, Triplexes and Quads by Larry Loftis. 

The gateway cities are going to be tough to find anything.