The problem is that while old prebuilts can be good PCs to *use* on a very tight budget, old prebuilts are bad for *upgrading*. Whether it's an Optiplex or any one of a number of branded prebuilts, they don't really have a good foundation for building anything on them. We're usually talking some combination of proprietary case and motherboard and power supply, usually all low quality and with the cheapest RAM they could find for the spec. So you end up having to buy a new case and motherboard and RAM anyway, and many of those prebuilts have dinky little versions of the GPU with the barest bones cooling solutions possible.
If you're hoping to build something long term starting with a very small budget, you need to scour for bargains on aftermarket parts on eBay or elsewhere and build a rig with parts that don't suddenly become unusable once you upgrade a different part.
You may get decent play from the Optiplex. But at that point, you basically have a disposable computer and when you want to upgrade something, you'll be back to zero for many of the big ticket parts.