Here is my perspective on the offer, from a numbers perspective only. Many others have pointed out the intricacies of the deal etc, and admittedly I have no experience in that area.
I *assume* that if you don't exercise the lease option, the option fee and the rent credits are both *forefeited*, which is my understanding of a typical lease option.
TAKING THE LEASE OPTION
If you take lease option, at $390K, first five years:
Monthly payment = $2600 (funny, that - see below).
(Assume CPI 3%). Payments are then $2600 (Yr1), $2678 (Yr2), $2758 (Yr3), $2841 (Yr4), $2926 (Yr5).
After 5 years, total payments (allowing 3% CPI) = $165,636.
HOW MUCH $$$$ do you pay up-front for the Option? This forms part of the 5-years costs. I haven't considered that here.
BUYING TODAY
If you bought today, at $370K, P&I loan 25 years, assuming 6.5% interest rate:
Calculated Monthly payment = $2600.
After 5 years, total payments = $156,000.
COMPARISON
After 5 Years on P & I loan at standard payments your loan balance would be: $327,890.
After 5 Years on P & I loan at the same payments as the Lease Option, your loan balance would be: $317,274.
After 5 Years on Lease Option you can buy for $ 390,000 - $39000 = $351,000.
If you could *buy today* at $370K and apply the same payments as for the Lease Option you would save yourself $33-34K roughly over the lease option. Note that loan costs and purchasing costs aren't taken into account here because they will be roughly the same whether you buy today at $370K or in 5 years at $351K.
Another question is who is responsible for rates under the lease option?, since as a tenant-buyer you would presumably not be liable whereas as an owner you would be. Building insurance, land tax (?) and other charges fall into the same category and haven't been factored in here.
Other considerations
What is the growth rate of the suburb containing the house? This is the critical factor. Let's say it averages 5%. In 5 years time the property will be worth $472K roughly ($102K increase). At 1% growth the property is worth only $389K after 5 years ($19K increase).
If you cannot afford to buy today, and instead choose to wait 5 years time (but do *not* take up the lease option and simply remain an "ordinary" tenant), you pay only market rent in the meantime (ie. $450 per week with CPI increases) so in comparison to the lease option you have "paid out" $39,000 less than with the lease option - I'll assume you "save" that money instead.
The $39K you save over 5 years will actually earn interest, so at 4.75% interest you'll have $44K after 5 years (you'll actually have to pay tax on the earnings but I've ignored that).
Assume 5% growth:
If you buy today at $370K: in 5 years house = $472K, loan = $317K, no savings (all paid into mortage), net worth = $155K.
Lease option: in 5 years house = $472K, loan = $351K, no savings (all paid into lease option), net worth = $121K.
Wait 5 years: in 5 years house = $472K, loan = $428K ($44K saved used as a deposit), net worth = $44K.
Assume 1% growth:
If you buy today at $370K: in 5 years house = $389K, loan = $317K, no savings (all paid into mortage), net worth = $72K.
Lease option: in 5 years house = $389K, loan = $351K, no savings (all paid into lease option), net worth = $38K.
Wait 5 years: in 5 years house = $389K, loan = $345K ($44K saved used as a deposit), net worth = $44K.
My conclusion
Buying the property today is always going to be the best solution so long as there is a modicum of capital growth (eg. even 2%).
If you can *absolutely commit* to buying the property under lease option, then it is still better than not buying at all, in terms of increasing your net wealth, *unless* there is 1% growth or less, in which case just saving your money and buying in 5 years will result in a bigger increase in your net worth.
Waiting to buy later (and not committing to the lease option) with even a small amount of growth means you cannot hope to save as quickly as the property is increasing in value. This, of course, is why most people borrow to buy their homes - because usually they have no hope of saving at a sufficiently quick rate (whilst still renting) to outstrip the property growth rate and "pay cash" for the property sometime in the future.
Finally, if you take up the lease option and *do not exercise* that option, you are more than likely pissing money away. At least $39,000 plus your option fee. So, whilst this is called an "option" I don't really think of it that way. The *only* time it will be of some benefit to you to not exercise the option is if the property market collapses. For example, if the property on which you hold the option is suddenly worth $300K in 5 years you get this comparison:
Had you bought today at $370K: in 5 years house = $300K, loan = $317K, no savings (all paid into mortage), net worth = $-17K.
Lease option: in 5 years house = $300K, you don't buy the house, no savings (all paid into lease option), net worth = $0K.
Wait 5 years: in 5 years house = $300K, loan = $256K ($44K saved used as a deposit), net worth = $44K.
In this case by not exercising the lease option you are not as badly off as the home buyer (not hugely different, however). And this example is based on a 19% drop over 5 years. In this case waiting would have been the best option, therefore buying cheaper later.