My plan is to take that slow train from Pattaya to Bangkok (Hua Lumphong Station) then take the second class overnight sleeper down to Surat Thani. It looks like the train from Pattaya leaves during the week at 14:30 takes about 3 hours. Anybody recently made the connection at Hua Lumphong heading to Surat Thani? My goal is to catch the train # 85 departing at 19:30. It looks like I have plenty of time at the train station to get my ticket. Any thoughts on same day travel/purchase and if the southbound train might be sold out?

I wouldn't take the risk when it comes to making a vital connection . Anything can and usually does happen with Thai railways.

You better off taking a bus or minivan to Bangkok and will have far greater peace of mind

I did the same a few years ago when I was taking the overnight train to Chiang Mai. I enjoyed the slow ride into Bangkok, local sights and smells. I recommend it.