I would suggest going with what you are comfortable with as a DM so 3e.
1e differences
static saving throw targets based on victim level and whacky save categories.
Classes and races were not balanced.
Races were restricted in what classes they could take and how high level they could go.
Multiclass overlapped instead of stacking and split xp between the classes.
Characters had to stick with the same class(es) forever unless human then they could multiclass like 3e with significant restrictions.
Classes had different xp charts.
Some classes had stat requirements to join (paladins required 17 charisma or something like that).
No CR for monster evaluations.
Nonhumans had infravision, seeing heat, enough for IDing bodies in the dark but not reading.
Only thieves had skills.
Combat rounds were a minute long, initiative was rolled every round.
AC and attacks were backward from 3e math but still started at AC 10.
Attacks and saves were in the DMG, not the PH, players sometimes had no idea about their capabilities.
Stat bonuses were not as regular as in 3e, usually 16 was the least for any bonus so 18s were super important and str 18 gave a percentile score for fighters with higher bonuses.
No feats.
More alignment restrictions for classes (druids must be neutral neutral and rangers had to be good).
magic item costs were wacky and nonstandard.
PCs got xp for killing things and recovering loot.
Wizards (called Magic users) did not get bonus spells.
Item creation was tough, potions and scrolls were not easily available.
After 9th level every class got a flat number of hit points per level based on class.
Classes restricted what armor and weapons a character could use, no penalties for non proficiency, they couldn't use them at all.
Poison did not do ability damage, usually save or die or hp damage.
Energy drain was no save, just lose the levels.
No AoOs.
No separate touch ACs.