I took the Buquebus last month, though the Montevideo to Buenos Aires route via Colonia.
I’d recommend arriving at least 2 hours early. I had a large suitcase, a large rucksack, and a medium backpack and nothing was said to me about it being too much luggage. I didn’t see the first class area but I doubt it’s worth it, the standard seats were comfortable enough for me, even with following 15 hours of travel from Europe
Sorry, just realised I took Colonia, not Buquebus. I hope my feedback can be helpful in any case