This thread again.

Well there's something better out but it's closed source and might not survive: BB OS 10 & Z10.

Consequently, I'd still recommend a sub $100 used N900 (or N9) with an eye on Sailfish.

The N900 especially needs a lot of setting up and you'll need a lot of community help with that. The memory capacity is relatively tiny which is a real shame considering Maemo multitasks really nicely.

Re: overclocking, your N900 is going to be seriously outdated by the end of the year. Don't worry about it.