I'm thinking about water heaters. 
Mine blew.  20 yo tankless Rinnai
My gas boiler is 25 yo and worked just ok but... 
Do I replace the WH only... $1,200 + install??
Do I replace the WH and Boiler with a nice Combination Unit ... $3,000 + install? 
Rheem, Rinnai, Peerless, other??
Ima going to DIY the thing.