Revisited: Spa (Hot tub) Renovation - Gas?

First, thanks to all who responded to the previous thread. Highly educational.
Here, I add a little data, and correct a few bloopers.
I've just inherited a working 8' wide spa (AKA hot tub), fiberglass body in a 2" spray-on-foam-insulated wooden frame with an insulated lid, holding about 500 gallons. It's an Apollo Spa # AP 96960, with a pack that's clearly OEM'd from elsewhere, a CM101 28-Z132NVVXS-AP, serial BA97101-010. Apollo is up in Pugetopolis, and that's where my nephew bought in when he moved to Seattle, eight years ago.
We're going to install this outside on concrete, on what was a small (non-reg.) basketball court. A windbreak or polyfilmed gazebo will surround it.
My county's building code requires a separate circuit to feed the 40A 240V draw of its control panel (AKA 'pack'), dual-stage pump and electrical heater. Installing that circuit, and the new panel it would require (as the existing panel can't handle it) will cost $2,400, as per two estimates from reliable electricians.
A hot tub manufacturer's website estimates that at 45 degrees north it will cost $380/year a) to run the pump on low by timer to keep it clean, b) to run the pump on high when we're in it, and c) to heat it to the temp most folks like. That assumes our electrical rate is at its current Enronesque 8.4 cents per KWH.
Has anyone here reworked a similar hot tub for gas heat?
One issue raised by Apollo when I calle dthem was the greater longevity of electrical heater elements and lesser replacement cost, as opposed to gas heat exchangers. Any comments? Bear in mind the electric elements have already eight years on them.
Natural gas and LP gas are much less expensive to heat with here (our electric utility is owned by Enron, happy, happy, joy, joy), and if I can replumb it to use an on-demand heater, http://www.realgoods.com/renew/shop/product.cfm?dp "00&sd"04&ts006973 the 36 KBTU heater would cost less than $ 800. It also would reduce the size of the electrical install; I wouldn't need a separate circuit and therefore would not need a new electrical panel.
A 150 gal. propane tank install would cost $220, with an annual tank lease of $70. Refiils of propane now cost $1.63/gal., and 4 gals of propane (assuming 68% efficiency) will bring that 500 gal tub up from 55 degrees F (groundwater) to 105 degrees F.
I am trying to find if the heater aforementiond can work with natural gas. Rheem has several (AKA RayPac) for natural gas for about the same price, but do not have the thermosiphon capability, and automatic operation. Here's a 50 KBTU model: http://www.pool1.com/detail.asp?product_id=RAY-15-454
It would be nice to use the existing control system ('pack') but those can be had for <$600 http://www.spadepot.com/Merchant2/merchant.mv?Screen=CTGY&Store_Code=SD2000&Category_Code=Packs and could allow us, with pump replacement, to go 110V completely: http://www.spadepot.com/Merchant2/merchant.mv?Screen=CTGY&Store_Code=SD2000&Category_Code=pumps
I think if I can drop the load to 16A (control pack plus pump for jets and filer), the electrical install will be much cheaper. A new estimate will be requested, to ask that question, but is there any reason why 110v is not preferable to 220v?
Also, what kind of insulation's good to add to space between the tub and the wooden frame, and underneath between the bottom and the concrete?
Sadly, Real Goods is not able to tell me if their wood-fired heater http://www.realgoods.com/renew/shop/product.cfm?dp "04&ts000350&kw=chofu is Oregon DEQ certified. Since we live in the upper Valley, that's important.
Sadly, it looks like the solar option from Real Goods cited in my original posting would not have a decent cost-benefit. We may look at that later for internal hot water heating.
So, your comments regarding converting this spa to gas would be appreciated. Thank you kindly in advance.
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00&Category_Code=Packs
00&Category_Code=pumps
Have you looked into these heat exchangers, which will heat your pool or spa from your existing boiler?
http://www.hydronics.com/Triangle_Tube/maxiflo1.htm
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