Was thinking on buying a 2001 mercedes e320 4matic wagon but it has a bent subframe is it worth buying

To be honest it would be cheaper and a lot easier just to buy another car with a smaller engine in it!
It is possible to change but unless you have a lot of experience working on cars and the current engine is beyond repair it isn't worth the work needed.
You might have to change engine mounting positions, drive shaft lengths, subframe and mounting points. Then there's the electrics, a totally different wiring harness might be needed as well as any changes the the ECU on the vehicle

When it comes to chassis and sub-frames it is ALWAYS safer to replace! They are structural parts so safety first.

buying any second hand car is a risk of loosing your money
check for rust in the body and subframes as if the shell is no good , then it will be a waste
take the car to someone that does car inspections and get a report on the condition and then decide on what you want to do
timing belts here cost around $900.00 aud to replace as a good mechanic will do the belt , replace idlers and adjusters and the crank and cam shaft seal/s
oil is very bad for a belt so if the seals are not done , expect about 1/2 the distance on a new belt
at that distance , it will be on the second belt and to make sure, all that I have referred to should be replaced

All it takes is money and a good frame shop. Anything can be done if you are willing to pay for it.

Is the belt to the power steering pump OK and if so is it slipping hence loss of steer assistance.

CHEAPER BUY WHOLE CAR AT SALVAGE YARD USE THE SUBFRAME.SUBFRAME CAN BE WELDED AND REPLACE IN SECTIONS BUT YOU WILL HAVE TIRE ALIGNMENT PROBLEMS.YOU CAN CALL AROUND GET A PRICE CHECK.YOU GOING TO BE OUT OF $1000.00 OR MORE. BECAUSE WHOLE ENGINE AND TRANSAXLE DRIVE TRAIN HAS TO BE REMOVE OFF SUBFRAME BEFORE REPLACING IT.

It's cant say any particular thing as every thing are very important, so best way to do talk with owner and get the car to Mercedes dealer workshop which is the near by your place and tell service adviser your requirement then they will give you a very advance report that's the best way to do it becouse if any simple thing wrong it will cost you too much money for fixing, if you spend little money for checking it worth for you than spend big money for repair, Is this information helped you?

IF SUBFRAME BENT ALL DRIVE TRAIN AND FRONT SUSPENSION COMPONENTS GOING TO BE KNOCKED OUT OF LINE.

There is NO open recall on the Santa Fe, However, there is one on the 1999-2004 SONATA (EF) and 2001-2004 XG300/XG350 (XG) vehicles that are registered and operated in specified “salt belt” states may exhibit corrosion damage to the subframe assembly. If specified levels of corrosion damage have occurred as described in this bulletin, replace the front subframe with a new subframe that incorporates additional holes in the upper and lower panels. If a front subframe does not require replacement, add drainage holes and treat the subframe with rust-proofing material.

a/c control unit or low refrigerant pressure-leak in system,find leak and get it fixed,dirty oily spots on a/c lines or at sealing points