Buy the most comfortable chair if you decide to up size. You will be spending hours drilling the tangs (providing that you have enough metal to have at least the circumference of the new hole size, not to mention the additional width required to share the load) without compromising clevis pin loading.
Mechanical fittings are going to cost a bit at the front end. If you install them correctly, they can be reused.
I would go for an open body turnbuckle - stud to toggle, you probably have old Navtec turnbuckles which do not work in rerig situations.
In the end, you are going to pay more for mechanical fittings, gall a few in the learning process, and become exasperated over the whole ordeal.
What price were you quoted for an entire rerig that you thought was out of line?