You are sitting on the edge of needing something in the 50hp range, IMHO.
True, right now you can't see exactly why.... but you will note that several posters indicated that they started at about 30hp then moved up.
You've got 100 acres and, typically over time people develop needs to do things with the land.. and a bit larger tractor is needed. At age 50, you've got energy and ideas and want-to.
Unless you are wanting to simply maintain a wildlife sanctuary, I predict your need will develop for a tractor capable of working harder/faster/stronger than a 35ish HP .
Given the land, horses, etc, I'd spend your money on these things

:
4 wheel drive
FEL
QUICK ATTACH/SKID STEER ON THE FEL
HYDROSTATIC TRANSMISSION (you can't believe how precise it is and safe)
HAY SPIKE FOR LARGE BALES
note, other attachments will suggest themselves.... observe the variety that other TBNers are using! So, begin putting aside $ for that.