Not sure what the "limited" warranty is you write of. Yes they have a 1 year all parts/labor warranty which is one year less than others but I can tell you from experience, if I hear from a customer on a Raypak, it's at least 7 years down the road from the install. I've replaced older Raypak's that were in the field for 10 plus years and it's the first time it was ever touch bu a repair tech. All manufacturers will NOT cover any warranty repairs if water chemistry is the issue. Pentair which is the same at Sta Rite, not the better unit on the market, my opinion.
I have been in the business for a long time and work on all heaters. I see fewer calls on the Raypak's than any other model in the field. I install Raypak's over any model due to what I see in the field. The last thing I want is an unhappy customer with call backs for issues. I used to install one other brand some time back because they were the best at the time. However they started failing me and I has numerous issues and unhappy customers so I stopped installing them. If you look thru this site Raypak gets the nod all the way around and not just from users but from the professionals on here like me.
I would also encourage you to use a installer that is properly licensed in your area and has knowledge of gas appliances. You mention a shed that you have your equipment in. If you choose to install the heater in there, make sure it is vented properly as well as that their is correct fresh air so the unit operates correctly.