Welcome to the boards and what a nice grandchild you are! I just turned 60 myself but my grandbaby just turned 1 the week after my own birthday.
I'm with the others in letting grandma choose. Just like some people like some brands of TVs or Cars, some of us do have preferences. Plus what we do/what we already have makes differences in the style of machine we choose. Needs for heavy duty can include quilting (where we want a large flat throat area) or for upholstery or garments (where you will probably want a free arm) or embroidery which is a whole set of choices.
I don't know how close you are with your grandma, but I would really enjoy the time that talking about the choices and looking into things together would bring. Even if that is done long distance/virtually, it is time together. Grandmas understand that people have budgets, and really it is the thought that counts.
People are often surprised by how much a sewing machine can cost. As a comparison, look at the costs of TVs or computers. Yes, you can get something usable on the low end but the high end is in thousands and not in hundreds. I'd say that servicing/shops don't really matter for a machine under $500, if something major breaks you just get a new one. I'd be happy to shop and get the lowest cost including shipping.
edit: I was given a modern Bernina 820 model that was hugely expensive in the beginning and now after 10 years and 2 previous (unhappy) owners it still has a going price of $2-3,000. In good news, both previous owners are deliriously happy with their 770s, and I like the 820 just fine. I did have to pay for a repair/cleaning and was very happy when the bill came in under $200. Models like this should also be taken to a shop and professionally cleaned (like a car tune up) about once a year too...
One of the brands not mentioned is little known, but a growing favorite of quilters especially those who don't appreciate computerized everything.
https://www.eversewn.com/