BaldLoonie
The RAPM-060 is 14.50 SEER with any blower.
At 13 years old in Dallas, you should have some life left in a Trane furnace. So up to your budget whether you want to replace it. With the reliability of today's stuff, you might be better off with the old furnace, sad to say
As for DIY, unless you have the tools and training, not a wise move. Hard enough finding a good contractor to install something right. Most nightmares of jobs are poor installation. Buying off the net usually means no warranty and I'd sure want a warranty with my equipment.