Donation features. Especially ext+, fly, pyro/pyro+, water, multihome, teleport, etc. (I have water, fly, multihome, and teleport, but I also have extcommands the normal one)
The good rising towns are gone, don't bother. Don't play lotto. Don't loan money or rent out msword/eff7. I would rent a spawnshop. You say you have limited time online, well a spawnshop is perfect for that. You don't have to spend a lot of time gathering stuff to sell -- you can buy stuff for cheap from other shops and mark it up if you have an easy-to-use and attractive store design. Or, you can set up buy signs in your shop for a price higher than the server price and your customers can be your suppliers. All you have to do is take the product out of your Buy sign and put it into your Sell sign. If you don't have a good set of tools/armor/weapons, I would get one. Buy: Eff5 Unb3 Silk pickaxe, shovel Eff5 Unb3 For3 axe, pickaxe Eff5 Unb3 Silk shears Sharp5 Loot3 sword Prot4 armor set (with Aqua Affinity, Respiration3, FF4 on the helm/boots) In the long run, these things will save you time and pay for themselves. If you don't think you will use one of them, then skip it. The next best investment is donation features that save you time or help you make money faster. Fly, ExtCommands+, TP, MultiHome are great. Pyro+, ExtCreations, McMMOBonusPlus, /back are very good. Think about it and buy what will help you the most.
1: I would decide to spend my money at a more rapid rate, not spread out my $500k for a spawn shop over about a year and a half (trust me, i did the math (thanks google calculator, lol)). 2: I would probably hoard some items that would be worth way more money later on then now. I would choose something like nstars, or even an nstar tool, just in case ECC decides to stop creating them, and everybody else breaks theirs.
I didn't say to spend it all on the spawnshop. I recommend doing all of things I listed at the same time.
I would buy 4 plots all attached and make it a multi level pumpkin/ watermelon farm and sell perms to it.