Becasue of NDA it is actually pretty hard to give you any proper answers here. But let me try.
Firstly I absolutely would NOT recommend what you are suggesting. Your partner should by this point in time have strongly recomended against this process, and if not, then I would seriously question their motives. The product is not yet released because it is not inalized, therefore what we are seeing right now is a beta product It is quite likely to change by the time it hits the streets.
Also traditionally (sorry due to NDA I can not say more here) the upgrade tool kits are never released at the same time as a new version, that normally follows six months or so after product release. So there will not be any method of getting your 5.00 system upgraded in November to 2009. I would check that with your partner, because maybe the upgrade tool kit is planned to be released in November also and they know somethign that the rest of us don’t.
Generally early adopters to a new version of Navision, would start with the new version, and the implementation process, which might take 6-12 months starts form that version, so lets assume NAV 2009 is released in November, you woudl be expecting clients to go live in say May.
Now I am not saying that you wont be live on NAV 2009 in December after an upgrade of your 5.00 system that you implemented in September. i am just saying that its highly unlikely, and I would suggest you get some very strong terms in your contract with your partner if you really do expect this to be happening.
Personally my advise would be to continue as you are. i.e. implement 5.00 SP1, this is a great product, very stable and very nice to work with. Then when NAV 2009.1 or NAV 2009 SP1 comes out I would look at the upgrade path, and make a decision then to do the upgrade. Any other path and you are just asking for trouble.
Sorry if the above is not what you wanted to hear, but my whole career is based on recovering Navision systems that failed, so I do have a good idea what I am talking about.
PS why can’t you use 5.00 SP1, is there a particular feature of NAV 2009 that precludes the use of 5.00?