Straight lug speedies - 2915, 2998, 105.002 and 105.003; ref. 105.012 and 145.012 which had the cal. 321 and everything following that with the cal. 861 and later versions were twisted lug speedies.
Ref. 2915-3 was identical to ref. 2998-1 and some 2998-2 versions - they had the black base 1000 bezel, domed dial and alpha hands; some had lollipop seconds hand others did not.
As for what to get - I would get the earliest reference you can get with all original parts in the best condition you can find within your budget.