Like person above me, I just read everything that's given for each project and make a decision to go or not to go. Since all these projects have a property secured as a collateral, key parameters to look for is LTV (loan-to-value), but also absolute value of the property, which in a way tells you the risk. Everybody has a different risk threshold, mine peaks at 60% LTV, which is hardly ever triggered at EstateGuru. Absolute value of the property hints at its liquidity. Should a loan default, it's definitely harder to quickly sell off a 1mio EUR valued property than a 200K Eur valued property. The quicker a property sells, the faster you have part or all of your money back.
Offer may seem limited, a rough guess from experience, I think they launch on average 4-5 projects per month. What is important is to check your email, as they usually announce a project with one day notice, which in the world of SEPA gives you just about enough time if you are quick to transfer in money, so they sit in your EstateGuru account the following day ready to be invested.
Volume is big enough, following my above rule I am able to catch a piece of every loan I want to invest into 4 out of 5 times.
Strategy -> diversify into platforms, diversify into loans. How much in absolute terms really depends on how much capital you have. I try to invest at most 2% of my total P2P investments into a single loan, though I average at around 1.5%.