Have meteorites been found in the oldest sedimentary rocks?
A main source of meteoritic origin in sedimentary strata could have been likely to have been placed in a marine sedimentary environment as debris from a larger impact. Evidently, as you stated, the older the sediment, the greater the chance of alteration or even complete destruction of any trace of the fragments.

In addition to the micrometeorites found in Sweden (as per DavePhD's linked articles), the article Cosmic-ray exposure ages of fossil micrometeorites from mid-Ordovician
sediments at Lynna River, Russia. (Meier et al. 2014), report of observations of chondrite micrometeoritic dust occurring in mid-Ordovician (~ 470 million years ago) sediments found around the Lynna River, Russia, as well as localities in China.  The authors state that the composition of the micrometeoritic sample makes them largely resistant to weathering and diagenesis.

The authors of the article Noble gases in fossil micrometeorites and meteorites from 470 Myr old
sediments from southern Sweden, and new evidence
for the L-chondrite parent body breakup event (Heck et al. 2008) concluded that it was 'remarkable' that the solar gases trapped in the chromite grains from the Ordovician-aged 'fossil meteorites found in Sweden (and according to Meier et al. 2014, also in Russia and China) have survived not only the heat up due to entry into the Earth's atmosphere, but also the long residence time in the sediments.

Further back, finding pristine sedimentary layers, let alone intact meteorites within the strata become increasingly difficult to find, so there incorporation into the geology would be in the form of geochemical traces.  

A couple of examples of this include the following, the Nature article (Abstract only - paywalled), Tungsten isotope evidence from approx 3.8-Gyr metamorphosed sediments for early meteorite bombardment of the Earth (Schoenberg et al. 2002), suggest that Archaean aged metamorphic rocks found in Greenland and Canada contain tungsten anomalies that can, according to the authors, only be meteoritic remnants in the metamorphosed sediments.  Chemical traces of possible chondritic origin have been hypothesised to have been included in Archaen spherules in metamorphosed sedimentary strata in South Africa, according to the article Geological and Geochemical Record of 3400-Million-Year-Old Terrestrial Meteorite Impacts (Lowe et al. 1989) (Abstract only -paywalled).