Good luck going forward. The transformer that comes with this set, although I don't recognize the brand, should be sufficient for a 4' x 8' layout.
Things to consider: Do you just want to see trains go around and around? Or do you want to build in switching opportunities? Do you want a second inner loop or a passing siding? How soon do you plan to expand?
I am not familiar with the layout that you referenced.
I don't see switches in the box, so that might be your first purchase. Install them in the beginning even if they lead to no where. I would add a second terminal track, or learn to solder additional feeder wires at additional points.
Recommendations: Try to avoid taking the track apart and reassembling it with any frequency, as this will weaken the rail joiners and eventually affect the current flow. Many folks solder the rail joiners or the track. I do not because of potential expansion/contraction issues with the plywood. But I have power feeders connected to every track section. I use flex track so there's a feeder every 6' or so. Use as much flex track as possible, even on dead-straight sections to reduce the track connections.
If you are going to use DC for power, then a MRC power pack is usually the best choice. You will need a second one if you have an inner loop that you wish to control separately at the same time.
Replace the horn-hook couplers with Kadees as soon as possible. Exchange wheels for metal ones if you plan to stay in the hobby for long.
Learn about DCC and decide if it is right for you. I use DC because I am a solo operator, I can only control one train at a time, sound means nothing to me, and I don't want to pay for or install decoders in 80 engines! I do have multiple power supplies, but as I stated, I can only pay attention to one train at a time.
Post pictures as you progress since we all love pictures. Ask as many questions as you have, the only dumb one is the one that you don't ask! If you have the time, come join us in the Coffee Shop for both train banter and useless chatter. There's a lot of action there although it's not everybody's cup of coffee!