Is it legally tenable to withhold rent in the case of repairs resulting in unlivable space conditions in much of the place

Much depends on your lease.  You should have an experienced attorney review your lease to determine your rights. Of course, all leases should contain a "right of quiet enjoyment" provision and thus, you should not be forced to live in such a condition. As for your security deposit, there are stict laws in New Jersey concerning security deposits. A landlord is not permitted to take yoru security deposit under most circumstances. More specifically,  The Rent Security Deposit Act states what a landlord must do with your security deposit when you move out, even if you move out before your lease is over. Within 30 days after you move out, the landlord must return your security deposit and interest, less any rent you owe or any charges for repairing damage that you have done to the property. If the landlord deducts any amounts for damages or rent, he or she must give you a complete list of the damages he or she claims you did to the property and the cost of repairs. The landlord must send you the list of damages by registered or certified mail, and the landlord must return to you any money left over from your security deposit. Cite: N.J.S.A. 46:8-21.1. The landlord can only charge you for property damage that is more than ordinary wear and tear. Ordinary wear and tear means damage that takes place from the normal, careful use of the property. Examples of normal wear and tear are faded paint on the walls, loose tile in the bathroom, window cracks caused by winter weather, or leaky faucets or radiators. Examples of damages that might not be ordinary wear and tear are large holes in the walls caused by nailing up decorations, cigarette burns on floors, or a broken mirror on the bathroom cabinet. Landlords cannot charge cleaning fees to tenants who leave their apartments broom clean. Landlords often try to deduct such fees, as well as fees for painting. There are steps you can take to prevent a landlord from charging you for ordinary wear and tear, cleaning, or painting. Before you move out, ask the landlord or superintendent to personally inspect the apartment. Then ask that person to sign a note stating that you left the apartment clean and undamaged. If you cannot get the landlord or superintendent to inspect the unit, have a friend do so. Ask your friend to take photographs, and sign and date them. If you have a friend do this, make sure the friend can go to court with you if necessary. If you end up in court, the judge will not accept a letter from your friend as evidence.