First, lets get something straight. NONE of the things I mentioned are plants, they are all animals
Other than seagrass, anything that looks like a plant is either a macro algae or a colony of animals. That being said, all that I mentioned are in my own octo tanks with PC's (and at the low end of the wattage for the tank size). You will want some lighting but nothing I mentioned needs heavy duty lights or major formulas to have them survive (caveat species does matter though and always check the requirements before buying). I have one tank lit with a set of of aquatic (made for an aquarium) LED's and all critters are doing fine and have been using that light bar for over a year. If you can afford LED's that is the way to go but they are still way beyond my budget to start replacing my PCs (power compacts). I suggest shopping on-line, finding things you like, read up on their care requirements and then buy locally if you can find it affordably (this reduces the risk of adding too much at one time since ordering on-line is more cost effective to buy in quantity because of the shipping). There are some gorgonians that are not photosynthetic but most need to be place high in the tank with a lower lighted aquarium. All need a lot of water flow.
Starfish won't scratch anything
but urchins (I forgot to mention them, pencil urchins are recommended but some keepers have gotten away with pin cushions - I have them but not with my octos - they are very useful for cleaning up the live rock but will eat your coraline - the purple stuff most people want - algae) will some times but not drastically since they typically stay on the LR. The LR and sand are the worst culprits and you would not believe how easy it is to scratch the tank with either one. Also, cleaning is more laborious because you can't use a metal scraper but I like my acrylics and could not make a call which I prefer.
Another way to entertain yourself with your tank after the initial cycle (about 3 months) and before you are ready for an octopus is to visit your local Asian market and see what they have live. I have found regular clams on ice to do well from any seafood counter but you need to temperature acclimate them slowly and I let them sit overnight in tank water to remove whatever they have taken up while on ice (and to be sure they are alive and viable). The octos occasionally eat them but they can live for years without any concern. They are not attractive but help a little with water quality, don't pollute like other bivalve molluscs (oysters and mussels are nasty) if they die and you miss noticing, are an inexpensive addition while you are learning, can be a food source and can be left indefinitely in an octo tank. I have also found snails there that have lived over a year (you need to ask if they are fresh or salt/brackish water snails. If that doesn't translate, try one in fresh and one in salt before you put them in the tank). It is rare to find live octopus (lots of frozen) and the few members that have found them (only one in the US) were not successful keeping them alive (most likely due to the shipping method). I found a conch once that lived for almost a year but then died for no clear reason but I suspect starvation. Conch, however, is a natural food for octopuses and the one I brought home went into a non-octo tank. Lobsters and abalone are not a good choice as they would require water colder than your tank. I take the stance that anything in a food market is already destined to die, and if I feel I can try it in my tank with out causing it to suffer (i.e. I have a good idea of what environment it needs) I will make the attempt. With only one tank, you also need to consider if it can remain in the tank once your primary inhabitant arrives.
One of your bigger expenses is going to be your live rock, it is not cheap no matter how you buy it (you will need an absolute minimum of 100 pounds and preferably closer to 200) but finding a local aquarium club often leads to swaps or finding someone who is leaving the hobby. Also CraigsList is a viable place to shop for the same scenario. You don't have to get it all in the same place but you do have to expect additional cycle time each time you add new. Other than crabs and snails, you should have all your LR in place and finished your ammonia->nitrite->nitrate from the last LR addition cycle before adding critters. Even crabs and snails should not be added until you have cycled the initial rock for at least a month (validating by checking for nitrite). This can be somewhat mitigated by cycling the newest rock in a garbage can rather than in your tank if you happen to find some later in the build out.
A word of caution, it is a good idea not get a tank so tall the you cannot reach the bottom (a lot of the larger tanks come in "tall"). I highly recommend measuring your arm from the arm pit to the end of your palm and don't exceed that height after any canopy you might add. I have a 4' and a 3' tank and would not do that again. Most people find 2' tall pretty much the max height that allows them to easily (not always comfortably) clean the full sides of a tank.
The cube tanks are very popular right now and I believe you can find a 90. Don't worry too much about round numbers and calculations. A 100 gallon tank will not be filled with 100 gallons because of the displacement of your live rock. You will want the largest sump you can fit. You cannot keep the sump full as it has to hold its normal water AND all the water that will drain back from your main tank when the power is turned off. Keep in mind that your skimmer will be above your sump and you will need additional clearance to remove the collection cup so the sump height needs to take the skimmer height into account.
Waiting until summer gives you some time to study tank configurations and learn how a system goes together. The tank talk forum has a sticky (at the top of the forum) called
Tank Buildouts with links to some of the more complete journals on TONMO. Reading through the ones listed should start you thinking about what you want to design. There are other forums that specialize in tank set ups that would be worth visiting as well once you have a general approach in mind.