A solid color e-ink device is built to do one job well. You are buying it for battery life that lasts weeks, zero screen glare, and an easy way to read graphic novels, textbooks, or marked-up notes. If you pick a reliable model, it should comfortably last you four to five years before the hardware feels slow.
Hardware makers typically roll out new color panels and processor refreshes in the spring. That window gives you two good options: jump on the newest screen tech or pick up the outgoing model for significantly less cash. Expect a complete setup—reader, protective folio, and a replacement stylus nib set—to run about $380.
Remember that an intro rate is a firm deadline, not free money. If you still have a remaining balance when the 18 months end, you will start paying standard interest on whatever is left. Pay the $21.11 every single month so your balance hits zero before the window closes.