I first started in 2010, I bought Adora and She-Ra via eBay around that time. Since then I have managed to get everything via eBay and Mattycollector, mainly.
I've been really, really lucky with what I've managed to pick up on the secondary market in terms of price, but being the completest I am it wouldn't be possible to start a collection now. These things have just become too scarce on the market, especially in the UK.
So I would say the best year for starting to collect MOTUC was 2010.