I also would not buy MHP tickets if you're only going to be there for an hour or two. That's not enough time to really do much and you'll miss out on most of the party exclusive entertainment.
On the other hand, I wouldn't skip going during the "party season" just because there are parties. As others have said, you're likely to only encounter one or two parties, depending on when you go and which days of the week you plan to be there. DLR during HalloweenTime is a lot of fun. Haunted Mansion and Space have their Hallowen overlays, along with more modest overlays at Luigi's and Maters. There are lots of decorations and specialty treats. The biggest drawback is that DL nighttime entertainment (firework and F!) are usually limited to the weekends, so you'll need to include a Sat or Sun if you wanted to see either of those (normally Fri also have nighttime entertainment, but once MHPs start, there is no nighttime entertainment for non-MHP guests).
If you go before the MHP start, you only have a limited amount of time to enjoy HallowenTime (it starts on 9/7 and the first party is on 9/19). The first weekend is likely to be pretty busy as guests go to DLR to see all of the new HalloweenTime offerings. The weekdays might not be to bad (assuming no ticket deals or other special offerings). If you wanted to avoid MHP, then I would try to go after the first weekend is finished, but before the first party.