Just a suggestion for the stats and my humble opinion: I would go for the stats similar unit (that is tested), and you can save some precious games and discussions about if it is too powerful or too weak or discussions. Then if you would like to make it unique, think in single special unit to represent something or the essence of the unit, forget the details that is not 28mm.
a sugestion for the giant spider could be: the same as Tusks or elephants. And also... spideres moves fast, so once per battle could move 30cm....
Another posibility is that the spider could enter in woods.
By the way, great choice of miniature, I also love Reaper Miniatures range to include in 10mm games. Good prices and also good sculpting!