@Vincent Chen Forney where I live just 10 miles southeast of Dallas is a great area for buy and holds. It's about 8 miles from Mesquite in Kaufman County.
I bought a 1800sq.ft home built in 2014 for $170k last year and could cash flow $250 easily because of the low interest rates should I decide to rent it out. We've seen a modest 9% gain each of the past 2 years. I'm not sure how that compares to other DFW areas. I don't think you could get $500 in cash flow anywhere in In the DFW area.
You can find properties under $200k brand new that will cash flow very well around Forney.
I grew up in Mesquite and try to avoid that area whenever possible. It's really gone down hill, but that being said there are some decent neighborhoods scattered throughout.
I would agree that North Dallas would be hard to cash flow. Forney has a great development plan for the future. The small downtown area is getting a complete make over. It's a 20 minute drive into downtown Dallas, but definitely far away from the boom in Northern Dallas and Collin County.
I commute to Richardson and it's a 25 minute drive. They're expanding PGBT SH190 all the way to I20 which will hopefully spur more growth in the future. Definitely a lot further away from the booming areas of Frisco and other towns in Collin county. Just my 2 pennies worth hope it helps.