USD index resistance is an indication of a higher possibility of USD falling. Whether EUR/USD is the best buy depends more on the pair’s chart than the dollar index, plus the other USD pairs’ charts.
Bear in mind if the USD index has risen to meet a potential resistance level, its possible it got there through an uptrend. The most likely next event during an uptrend is a price rise, continuing the trend: an immediate reversal is the least likely outcome - but offering dramatic gains if you’re right. Maybe a good idea to look for agreement from other USD charts, wait and look for confirmation, set a fairly close stop-loss.