I would like to be able to configure what kind of train and buffering the station uses and use those to set the request and provide limits and well as requested amounts for goods.

For provides there should be a field to set items with count. For example 3x fluid wagon, 2x cargo wagon. This then sets the provider limit to the size of 3 fluid wagons and the provider stack limit to the size of 2 cargo wagons.

For requests there should be two fields. One for the request (stack) limit identical to the provides field. But also a second to set a default request amount. This you would set to for example 6x storage tank, 12x steel chest. Then when an item or fluid is requested with no amount set the default amount would be used.

The GUI for all fields should be identical and simply allow selecting any number of items with a count, where items are filtered for fluid tanks, containers, cargo wagons or fluid wagons plus a virtual item for units and stacks. Usually the limits would be set in wagons and the request limits in tanks/containers/stacks. But no reason not to allow the user to set the provider limit to 2 tanks or request default amount to 4 fluid wagons.

It might be more readable to split the 3 fields by fluid and item.

This is an interesting idea, along with the notes in https://forums.factorio.com/viewtopic.php?p=554581. I've been needing to do a significant rewrite of this to fall in line with the latest flib. I'll consider this when I undertake that project...

If you want help evaluating a string the player entered into an integer let me know. It can be done quite simple with recursive functions that evaluate while paring the string. It only needs to run on player input and inputs will be mostly trivial so speed is not a factor. It's not like parsing even a 100 character string would take any time.