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1) For the desired specs, especially the 300mA load, the SN6501 will be too tight, since it's max current capability @ 5V is only 350mA. Since we need to use a step-up transformer to account for drops across diodes and the LDO, we need more margin in the spec. For this reason, I'd recommend the SN6505 solution. It can provide up to 1A and therefore is an excellent solution.
2) For the transformer, do you know if you need basic isolation or reinforced isolation? For Basic Isolation (2.5-kVrms) you can use the following transformer from Wuerth : 750315371 and for reinforced (5 Kvrms) you can refer to 750315240, also by the same company. These are for examples only. You/your customer can pick a similar spec transformer from their preferred vendor.
3) For the LDO, you can use a TI TPS76350 or TPS76701QD, or some other TI LDO with similar specs.
For convenience, the SN6505 EVM and User Guide for the above solution (including schematics and full BOM) is available here:
Table 3 in the SN6505 Datasheet is also a good source for transformer part numbers (Page 24).
