Unless you're going to use it very frequently, AND it makes a
big difference in your eliquid...I'd personally rather spend $100 on new mods, tanks, more flavorings, etc. Heck, 500mL is only 145mL more than the size of an average can of Coke. It's not like you're making Liters of eliquid at a time!
The little $7 Norpro Deluxe Cordless Mini Mixer 5-Piece Set I bought on eBay should stir/mix 200-500mL! I regularly use it on some of my mixes I make in 125-165mL batches, and it doesn't slow it down
at all. Using it has also cut down the aging/steeping time of the few recipes I make that have custards in them. Now...in
just 3 days, they're as good as they used to be in 2 weeks!
Or...just buy a handheld immersion blender that are available for about $20-30 instead. It has a much sturdier construction, a strong metal wand, metal attachment, and does high speed mixing! It also
definitely could handle mixing eliquids by the liter and even more! I've used mine for making milkshakes, cream based soups of all kinds, dips, etc. and it mixes them all very easily!
You could buy either one of those above, and a crock pot ($10-15 at Walmart) to heat the eliquid in afterwards. Total cost would be less than $25 with Norpro set or less than $50 with immersion blender. Plus, you'd have a tool and a crock pot to use for other things in the kitchen, too!
But, if you've got money burning a hole in your pocket...who am I to say the little $7 Set or the immersion blender would satisfy your "need to spend" more?