Austin has teased getting in the ring for one more match off & on for much of the last decade, usually saying he'd be up for it under the right circumstances.
A match against Brock Lesnar would have to count as the right circumstances and it'd definitely be a big money match just for the novelty of seeing Austin wrestle for the first time in what would be almost 13 years.
As to whether Austin would be up for the match physically, there's obviously no way for us to know but you have to wonder if he really would be. He'd be almost 51.5 years old by the time the match took place, his knees and neck weren't in the greatest of condition by the time he left and there's well over a decade of ring rust he'd have to scrap off; at his age, there's always the possibility that it wouldn't happen. Then again, maybe he's in great in-ring condition as he's obviously taken care of himself and the extended time off may have helped ease the damage done to his body via the business. Regardless, he'd have to be able to pass the battery of medical exams he'd be put through by WWE so WWE can cover its ass.
As to the validity of Austin's comments towards Heyman, I very much believe it's a work. Austin and Heyman have been friends for a helluva long time and I don't believe that Heyman merely asking a question about a potential match with Lesnar would set Austin off. I think what we saw was merely example of some passionate promo work without Vince's scripted fingerprints all over the place. On the slim chance it wasn't a work, and by slim I mean beyond anorexic, it'd feel more to me like anger stoked bravado than anything else. In a real fight, Lesnar would most likely cream Austin; to my knowledge, Austin has no experience in martial arts, has no real combat experience, is some 13 years older than Lesnar with bad knees and a bad neck.
As to whether I'd be personally interested, I'm not sure. The novelty of the match itself is intriguing, but if Austin's not in shape and if he's just not able to go, then the novelty wouldn't hold up for me. Even if the match took place, one has to consider how Austin would hold up under Lesnar's stiff treatment; Austin's not a spring chicken anymore and Austin's age along with questions concerning his overall health are not only valid points, they're arguably the most important points regarding the whole situation.