Hi Mike,
Thanks for your extensive reply, also the information on the Deluge, very interesting!
Most of your arguments understood, still please consider a 3 row casing, you don't have to straight away put it full with modules but at least you have a bit of space to expand.
With either 2 or 3 rows, I would indeed drop any kind of sequencer stuff since you got the Deluge already. Use the Deluge first to save space in your rack. Once you got this rack full and you are ready to go for the next level then you still can consider what kind of sequencer should replace the Deluge but you will see that wouldn't be an easy task, so stick with the Deluge first.
You asked for another VCO that might go well together with the Plaits (I would go for the original one, better user interface, which is important, for me at least), I think the Make Noise - STO might be a good choice. I use these two (Plaits & STO) quite often together and I am with both more than just happy, they belong to my favourite oscillators.
Financially, consider to stretch your budget either now or on the short term, Eurorack is really fantastic but it ain't cheap! So don't fool yourself by believing you can keep this semi-reasonable... no it wouldn't ;-) At least not, if you are really going to "dive into Eurorack"...
But above all, just get a casing (or make it) and start with just a few modules (since you got already the Deluge) and then slowly start extending it while increasing your experience and then sooner or later you will find your way into it and of course you are always welcome to drop a few lines here so we can try to support you into your sonic Eurorack journey :-)
Good luck and kind regards, Garfield.
For review reports of Eurorack modules, please refer to https://garfieldmodular.net/ for PDF formatted downloads