Re: auto oil / pre-mix
I agree with dcramer to hook it up right away if you're going to. There's only one part you need - assuming you have a functioning VRO/OMS fuel pump - is the tank. It will come with the hose and electrical wire and connection.
As for affordability, new, it would be about $150 locally for you I'd imagine. Ebay prices for new is usually <$100, used $50-$75. Search for "oil tank" and something like Johnson or Evinrude to get started.
Hooking it up and testing function and alarms is easy to follow in the service manual, or if you're confused, just post here for easy step-by-step instructions.
The only thing you need to be aware of is that there are two basic "types" when it comes to the electronics in the tank (to signal low oil condition), and this relates to how the warning signals work.
The two types are "system check" gauges and non-system check gauges, all you need to know is which system you have and get the right tank. I think system check types are often labeled "1996 (95??) and newer". As long as you get the right type, the electrical connections will be either plug and play or easy enough to adapt with barrel connectors. But you can't adapt system check to non-system check and vice versa because that difference is in the actual electronics in the tank.
Get the newest one you can, with the solid plastic cap (not the cap with the clear plastic top), and make sure you're getting all the mounting hardware for easiest installation (although that stuff can be figured out).