Assuming all houses are already claimed, can a player purchase a hotel directly by paying the value of 5 houses in order to directly purchase a hotel?
You can only build a hotel if you have four houses on your properties.
You must build houses evenly. That is, if you have three properties, then you must buy 1 house for A, 1 house for B, 1 house for C, 2nd house for A, 2nd house for B, 2nd house for C, and so on.
You cannot buy a hotel on one property.
If the houses are gone, then you cannot buy them.
Building shortages are explicitly a part of the game.
BUILDING SHORTAGES… When the Bank has no houses to sell, players wishing to build must wait for some player to return or sell his/her houses to the Bank before building. If there are a limited number of houses and hotels available and two or more players wish to buy more than the Bank has, the houses or hotels must be sold at auction to the highest bidder.
The rules for building a hotel state the following.
HOTELS… When a player has four houses on each property of a complete color-group, he/she may buy a hotel from the Bank and erect it on any property of the color-group. He/she returns the four houses from that property to the Bank and pays the price for the hotel as shown on the Title Deed card.
Taking those two rules together makes not upgrading to hotels an important part of the strategy of the game; creating a monopoly.
It can't be stated often enough: the objective of the game is not to get rich; the objective of the game is bankrupting your opponents. The way to do that is to create a monopoly and deny them the resources they need.
For example, even if your opponent owns most of yellow, green, and dark blue, if you own light blue and pink with the maximum number of houses on each, you will have removed 24 houses from the stock of 32 in total. The only colour your opponent can now max out on houses and possibly upgrade to a hotel is dark blue, with Park Lane and Mayfair. But of course you refused to trade Park Lane with them and kept it for yourself, didn't you?