There’s no daily limit, or any kind of limit at all, on the trial version, so whatever’s going on with the OCR is nothing to do with being on the trial. It’s more likely a quirk in the interaction of your scanner software with DT’s OCR; I know if I queue up multiple documents for OCR from my ScanSnap I need to remember to save each one before I scan the next or the queue freezes.
There’s no advantage, and considerable disadvantage, in paying for Standard before the 30-day/150-hour trial period is up, since all you’re doing is turning off the Pro and Server feature sets, which you may as well max out before you finalise which version will suffice. In particular, the trial period is the only way to check out the much more expensive Server version without taking the plunge and paying for it; it’s a bit niche and you probably won’t have any use for it till much further down the line, but it’s worth taking for a spin while you have it for free.