You can buy an aftermarket plan via a 3rd party provider such as Hollard, Liquid Capital, Etc.
To give it to you straight, there is no such thing as "The Best" plan, and none of these plans will be as comprehensive as the BMW motorplan. I have yet to come across a plan that doesn’t have “Claim limits” for items, for example; If your Gearbox packs up under BMW MP, they will replace, no questions asked. With a 3rd party plan they have limitations for most mechanical parts (some are covered, some are not), and pay out accordingly. So, as an example, they might pay R45 000 of a R50 000 gearbox.
The best thing to do is to obtain a few quotes, and compare the claim limitations per item, and weigh it up against the cost of the policy, be it a monthly premium, or a once off fee. Also, something very important to consider (especially because you are in in the process of purchasing a car) is the waiting period on claims. Some have up to 3 month waiting periods, and I can guarantee they won’t cover pre-existing conditions.
If I were in your shoes Ill review the service history up to date and make a judgement call following that. The E90 330d has proven to very reliable and bullet proof even with some serious mods, IF it was looked after properly.
Service wize, depending on how savvy you are you can DIY filters, oil and brake pads quite easily. Buy the parts directly from BMW, and watch a couple of Youtube tutorials, you will save quite a bit on Labour. If this sounds too challenging, go to Ross Duncan in Midrand. Lastly, remember to service your gearbox @ ZF once you hit 100 000km.