This is old and I haven't updated or read through it for a while, but it should still do you plenty of good:
Peach: This is a fairly unique match-up for marth that will definitely require a change of style.
Approaching Peach:
Peach has a high priority dash attack that can catch marth's feet when short hopping or landing, so full jumped fairs work well. Walking away from peach and waiting for her to move, then attacking her is effective. Also, jumping backwards and fairing peach works well. Since peach's main move against marth when on the ground is her dash attack, expect it when approaching her. Run up and cancel the run with a shield as necessary. If she hits the shield, then marth can either grab or jump out of the shield and dair peach.
Peach has one of the best crouch cancels in the game along with Samus. If she crouch cancels, then dairs and grabs work well. One thing that must be learned early on is to NOT HOLD DOWN. Do not crouch cancel against peach and be careful with dtilt.
Peach has turnips (watch out for the stitched face!) and many peaches love to spam them. When approaching a peach with a turnip and if she attacks while throwing one, marth's nair works well in cutting through the turnip and peach. It's best not to catch any turnips because marth can't use any moves while holding one, and it takes marth longer to throw a turnip than peach. Instead, prevent peach from grabbing more turnips by dash attacking when she predictably pulls another. However, it is possible to catch a turnip on the ground and air by pressing A when it is about to hit marth. A safer way to do the same thing is to air dodge and press A, because then marth can't be hit if he misses the turnip.
Marth has a fthrow chaingrab on peach at early percentages up to about 20%. Take advantage of it and get in a fsmash. Sometimes marth can catch peach off guard and hit her with a tippered fsmash at higher percentages if she misses the DI.
Recovering for marth:
Peach will usually try to hit marth with turnips during his recovery. When thrown horizontally at marth, avoid the turnip by either falling without a side B, jumping, swatting it away with a fair, or grabbing it. When thrown vertically down at marth, move either further in or away from the stage and delay the up B as necessary to avoid getting hit. Sometimes it is inevitable that marth will get hit by a turnip.
Defending against Peach:
When defending against a peach, if she is coming to grab then dash dance away then back in to grab her. Perform extra dash dances as necessary depending on whether she is sidestep-happy. If she dash attacks, then shield. If she is floating with a fair coming, then get the heck out of there with a wavedash (out of shield), dash, or roll as necessary. Do not attempt to out-prioritize her moves. At most, marth can preempt her moves with a fair.
Edge guarding against Peach:
When edge guarding, peach will sometimes have a jump until she has to use her up B. Try to hit peach with a fair or bair so that she has to use her up B, because it makes her recovery more difficult (no air dodge). If she has her jump, then peaches will usually either air dodge to the stage or fair. Never fsmash at peach when she is recovering, because she will always air dodge and punish marth. Use utilt instead. Peach hates the utilt, so abuse it on her. If peach has already used her jump, then the best thing to do is to make sure she lands on the ledge and not on the stage. Peach has a hard time getting from the ledge to the stage. When she is over 100%, it is usually safe to fsmash or utilt her as she rolls or jumps up on stage. Otherwise, a grab or dtilt is probably the best choice.
Note: It is sometimes possible to dair or bair peach while she is recovering depending on her height.
On most stages, peach dies at 140-150% by utilt. On Dreamland she can live up until 170-180% from an utilt. Tippered fsmashes can be very useful against peach on most levels and lead to an early death for her. She is so slow that it is hard for her to punish fsmashes other than dash attacking before marth can get off the fsmash.
Good point though, advice is advice no matter where it comes from.