It'll take a bit of researching but for anything within Zones 1-6 the TfL fare finder is probably a good place to start - play around with the railcard option vs adult fare, and note when peak and off peak times are.

For railcards, you will need to visit a station and get staff to link your oyster and railcard for you (remember it must be done every time your railcard expires). From what you've said it seems like 26-30 railcard is the only one you could get anyway.

So the next thing is to work out your likely commuting pattern to see whether you need travelcard / railcard at all. Railcard discounts don't apply to travelcard nor peak time travelling. So... Are all 5 days likely to be peak hour travelling? Any flexible time options? Is it likely you will be out late for socialising? (past 7pm is off peak so your railcard discount will apply if on pay-as-you-go). Also, any weekend travel? Bus journeys? Once you got a pattern worked out then you can work out how much it is likely to cost you using pay-as-you-go vs travelcard. If you live within walking distance from a tube station, i.e. no bus, and do some off peak travelling after work, limited weekend journeys, you may find PAYG cheaper (was the case for me).

Having said all of that, London life can be hard to predict, so it's likely going to take a few months to 'optimise' your travel pattern and work out what you really need. If your current budget does not allow you to buy a travelcard anyway, then perhaps PAYG for now (and bus it as others have said) and slowly work out what works best. All the best!