...... what makes a good scenario for spearhead?.....
In brief; historical OOBs and objectives. Forces will not be equal or balanced and "winning" is a very nebulous concept.
It really shines when played as a big game with lots of bases played over an extended period. Having multiple players helps but 2 people can play Spearhead very sucessfully.
Scenarios designed for the (also excellent) Panzer Korps rules will work with Spearhead but you will need to reconstruct the lower levels of the formations as the higher command level of PK aggregates support weapons into units rather than fielding them separately as Spearhead. Simple task as the Spearhead OOBs booklet is extremely good.
For 1940 something like this will give you a good place to start:
http://www.lulu.com/shop/manny-granillo/ghost-division-race-for-the-channel/paperback/product-559zpd.html?q=&page=1&pageSize=4You can also look at scenarios for the Command Decision rules as these are nominally the same level as Spearhead (slight differences in unit organisations).
For which the gold standard is the set put out by MGK Military Simulations
http://www.wargamevault.com/browse/pub/15933/MGK-Military-SimulationsNone of these 3 rulesets are warGAMES like Bolt Action, they are not designed for equalised points based matches and do not work if played that way.