Flying Blue now has a dynamic pricing system. Therefore, the "cost" of upgrading isn't fixed; it depends on the circumstances. Only when you have an issued ticket will they be able to determine how much an upgrade would cost, if it would even be possible to upgrade.

Upgrading using miles on FB is a poor use of miles. First of all, although in theory you can upgrade from any economy fare class to business, it is much more difficult to do so than it is to just book a business class seat outright. In many cases, it just won't be possible to upgrade using miles; and when it is possible, you will end up paying far more in cash overall, and will use almost as many miles as if you had just booked using miles to redeem directly into the business class cabin.

I would therefore strongly advise that you abandon all thought of buying an economy ticket for cash if your intention is to travel in business class. If you are already struggling to redeem into J directly, then you are not going to be able to do so by restricting yourself to a particular economy ticket on a particular economy flight.

Upgrading using miles makes sense if you are stuck on employee/client tickets provided for you in economy; i.e. when you have no say in the ticket you are provided with. It's always a lottery as to whether an upgrade will actually be available, but at least in these cases, you have not paid for the ticket, and had no choice but to accept the ticket, and therefore seeing if an upgrade is available means you may improve a bad situation.

However, if you are fully in control of your itinerary - as you are in this case - and you want to travel in J, then the worst possible thing you can do right now is to buy yourself an economy ticket, as the chances are that you will not be able to "escape" from your economy booking.