I dont recall seeing too many throttle body gaskets failing on Altimas, but I have seen a lot of intake manifold gaskets fail on Altimas. I believe the allen bolts are 6 or 8mm. The throttle body is harder than some, but overall not too bad; keep in mind that coolant passes through it. If it is the intake manifold gasket, it's a real pain. About a 4-5 hour job. Early Altimas, like yours, used triangular shaped ports intake ports while later versions used oval-shaped ports. The reason I mention this is that the parts numbers were switched in some Nissan parts systems so if it is the intake manifold, you want the correct gasket. You'll need Nissan #'s 14032-1E400 and 14035-1E400. (Later oval port used #'s 14032-5B600 & 14035-5B600). Be careful not to break the ported vacuum switch that screws into the coolant jacket near intake runner #1, at the bottom of the intake.
You don't need to remove the head. I find it is a lot easier to do if you remove the oil filter and oil filter adapter. Make sure you get a gasket for the adapter. There is a main support brace from the block to the lower intake that must be removed. There is also a smaller brace that bolts to the bellhousing and the throttle body that needs to be removed. Remove the starter. Disconnect the linkage to the throttle body and the appropriate throttle cables, fuel lines and vacuum hoses. Unplug the oil pressure switch and P/S load switch, etc. The air box/MAS/intake duct assy. must be removed. I think there's an EGR valve and/or EGR passage tube that has to be disconnected. The fuel injection rail must be removed. Coolant drained. Unbolt the lower intake plenum and let it drop down. Unbolt the upper intake manifold. Double nut the two intake manifold studs and remove from the head. *Note: some of the nuts are difficult to get to, so make sure you have a good assortment of extensions and 10 and 12mm sockets and universal sockets, preferably in both 1/4" and 3/8" drive. you should now be able to push the intake back far enought to replace the gasket. If the plenum gasket was not damaged, i would recommend re-using it. Otherwise, you'll have to agonizingly scrap the old one off and use a mirror to make sure you get all of the old gasket off. Reassemble and torque bolts to specs, working center to outermost nuts. Easiest done on a lift.
It is a lot of work to get the intake off of an Altima...Thank the engineers!
Intake nuts/bolts are 12mm. You'll see a lot of 10mm. The EGR pipe requires a 1-1/16th inch open end wrench. Philips head screws for the fuel hose clamps. You'll see some 14mm as well, as in the starter bolts. The starter is a real pain in itself, but I think you already found that out! You'll also need some universals and a selection of extentions...a razor blade or two...a scraper...Pblaster...a new oil filter and a wrench to remove the old one. A flathead screwdriver for the radiator drain. You won't run into any 13mm unless someone was in there before!