Have you ever rowed indoors before? It’s incredibly boring. You might want to consider your boredom/torture threshold!
The Opti rowers looks pretty cheap and nasty. Decent ergs are very heavy but need to be because you are generating a lot of power. Otherwise they’ll rock around a lot. I think you do pay for what you get, a decent Concept 2 is long and a new one is £800+. I would definitely want to try one out before buying, although distance selling regs might mean you could take it back. You’d work out pretty quickly if it was dreadful. I’ve not been involved with rowing for 15 years now, but i never saw any other make of rower in any boatclubs other than Concept.
You can get ergs for canoeing. At least, I saw one once at the rowing lake. Weird! Wont you be a bit claustrophobic exercising in such a small space?