That is a terrific question.
It depends where your passion is. You may want to specialize in collecting just one brand, or broaden your interest to many. You will also have to consider time period...do you want the very old 1800 machines, or more modern ones. You can collect the best of the best, or the obscure and forgotten. American made, German, Italian, English..the list goes on and on. Do you treadle, or like electric machines. Give it some thought, and then we all can narrow down your selection. I started with a Singer 301, and now have machines from all time periods and countries. I have found that I like the Bernina's, but beyond that I love the very old like the Singer 12 and Willcox and Gibbs, Davis, Pfaffs, Elna's.....the list goes on. There are so many great machines out there, consider narrowing it down or you will end up with so many you'll open a museum!!!
Just some initial thoughts. PS, take a look at the machine gallery, it may inspire you. Good Luck and we'll be here to support you.