Yeah, so the silver version is a nice looking device of the era, but as for functionality.. meh. The speech module is really the best claim to fame of the machine, and if you can get one of those, there are a few games that you can play with speech. As well, The Extended Basic cartridge will allow you to utilize the speech on your own, which can provide you with minutes of enjoyment. The games/graphics on this device are probably pretty similar to what you'd get with a Vic-20 and the games are very much that quality. To be fair, if you have a C64, theres nothing you'll want to do on that TI99 that you wouldn't rather do with the C64. This is definitely a nostalgia only device, meaning you'll only appreciate it if you already owned one back in the day (much like sinclairs).
For software, you'll want to look at the FinalGrom cartridge (or the slightly less featured Flashrom99). There are a few other add-ons but I have no experience with those, and again, if you don't have the nostalgia of this device, you'll likely not want to put in the time and expense to 'max out' this machine.