Velux hot water system

Jerry, most SHW panels are basically the same. Copper on copper or copper on aluminum in an insulated aluminum box. Check out the selection here. Try to buy local to keep our economy going. The rest of the system is just plumbing. Looks like you are thinking of installing on a roof. Last resort in my opinion. Keep it simple and keep it cheap for minimal maintenence and faster payback......Glad to see you reconsidered buying evacuated tubes. As you said more money, hard to replace old models and can't be repaired easily.

Jerry, the only way I recommend using solar for heating a hot tub is to draw the water from the drain of the tub and return it back into the tub using a hose. This way here you do not mess with the manufacturers plumbing that could void warranties. You can pump the water directly from the drain into SHW panels or into a heat exchanger as you mentioned. If you want to incorporate it with solar for domestic hot water,you will need a double heat exchanger tank.

Jerry, I would recommend flat plates. If you ever add to the system, it's a lot easy to match and connect flat plates. Evacuated tube designs are still changing to rapidly.
 Is the front of the house and the roof the only location you have to install the panels? Where is the hot tub and other utilities located? How much piping will be needed? Is snow a problem where you are located? Is there room for expansion? Can you add a second, cheaper hot water system dedicated to the tub? How many panels are you planning on using for both the tub and domestic hot water?.....
 Just some other questions for you to think about when installing a SHW system.