Welcome! You came to the right place to learn about your concerns.
Tear stains- if this is such a concern, you may have to consider another breed. I carefully clean my pups "discoloration"and debris twice a day without coming near the inner canthus or eye ( to avoid tension on the third eyelid and causing cherry eye). For me, I include cleaning the fur near his eyes in his twice a day upkeep of brushing his teeth ( buccal and palatal surfaces). Yes, Havanese need a bit of upkeep.
The puppy cut is great and I keep mine this cut, but I also do my own grooming and have done this for over twelve years (now on my second Hav). Even for me, at times this is very time consuming. I do a scissor cut with various types of shears and it takes me 2-3 hours every week to week and a half. It may take others less time, but this is not always as straight forward as it seems. And yes, texture and size of pup are both variable and affect the grooming. Nails are a more ongoing upkeep for my pup since I use a crystal nail file every 4-5 days. My current pup did not deal with nail clippers despite treating etc. The crystal nail file has been a life saver for us- and I get closer to the quick than with clippers. My pup is perfectly happy to have his nails filed with a few treats
The color does change and size is not always predicatable. My current pup had parents around ten pounds but he has grown to closer to 15 pounds. He is longer in the body than his parents
Not sure if you have had the privilege of being a prior pup parent, but you will find that being focused on the external aspects are not at the center of the relationship. As mentioned above, this relationship is all about unconditional love and the inherent self-sacrifices it necessitates. And that is the love that drives all of us on this forum to be so crazy about our furbabies. Havs will also challenge you in ways you don't anticipate, so it is best to approach this choice with an open mind. Preferences are one thing, but accepting the pup for their unique ways even if they do not fit into your expectations will take you on a wonderful journey with so much to experience and learn. They teach us how to open our hearts