I am an investor in small income producing properties (1-4 family). I know what my motivations are, and I have a decent idea of what the motivations of other investors are not only in my market, but around the country (check out Bigger Pockets sometime). I have found not all investors invest the same way and for the same reasons. I think the Income Approach is a bit basic and considering it assumes the price paid is a function of rent, is basing it's whole thing on that big assumption. Some investors purchase for purposes of appreciation. Some purchase for income. Some purchase so others can pay for a property for them. Some do all of these things at once. Some investors purchase with their own cash. Some investors use others cash. Some investors do both. All of these have an impact on what an investor can and is willing to afford. I don't think the GRM indicator is a "poor" indicator, I just don't think it always tells the whole story, just too many things going on to say what is probable. The direct sales comparison approach however is quite handy. This pretty much cuts the bull and gets down to the bottom line of it all. All investors compete with each other for property at this basic level, no matter their expectations, motivations or strategies.
For me personally, I purchase per unit. I have a certain amount per unit I will pay and yes, the ability to command rent is a factor in that (money in/money out). Also a factor in that are capital improvements, both what needs to be done immediately and what the property is expected to need along the way. As far as bedrooms yes, there is a bit of a premium that can be commanded going from a 1 bedroom to 2, and again from a 2 to 3. However, I don't really like 1 bedrooms because they are harder to rent at a higher price and attract single people who have a harder time paying and I don't like 3 bedrooms because the small amount of extra rent goes out the door with the extra wear and tear from larger families. I don't really care about the total number of rooms, so long as a unit has an acceptable amount of space for tenants to do their thing. Larger properties are actually a turn-off, as it's just more sf to maintain and repair. The best deals are often the little ones. But all that's me.
Some folks accept rent assistance and in that game the difference in bedrooms, paid by the government, not the market, can be substantial. It might make the difference between purchasing a property at a certain price point at all.
I buy in the city, but live in the suburbs. I would never invest where I live, there is just no money in it. The people who invest out here pretty much are all banking on either appreciation or getting someone to purchase a property for them, not about cash-flowing. Many of these out here have owner-occupants, as again, there is no money in them so they have to pay for them partially by themselves anyways, and it is cheaper to be an on-site landlord.
It really is a matter of different strokes for different folks.