Have 98 bmw 740i. Needs rear wheel bearings--how complex a job is this to replace and any useful tips thanks

Suggest you post this question on one of the BMW on-line forums, such as bimmerforums.com or bimmerfest.com, where you are likely to get more responses. However, depending on your skill as a DIYer it's not that big a job, but does require a special wheel bearing puller tool (at least the 3-series does). You can get a sense for what's required from this article on wheel bearing replacement for the hold E30 3-series:
Pelican Technical Article: BMW E30 3 Series Front Wheel Bearing Replacement (http://www.pelicanparts.com/bmw/techarticles/JF-Tech/BMW_E30_3_Series_front_wheel_bearing_replacement.h tm)

Personally this is one of those jobs that I would leave to a local independent repair shop.

Fairly difficult and requires specialized tools. This procedure should work on your 1998 BMW 740I:

1. Hub Removal. Use a slide hammer to separate the rear hub from the wheel bearing.

2. Hub Inner Race Removal. When the hub comes off, the front half of the inner wheel bearing race will be attached to the hub. At a compound 45 degree angle, use a 3-inch air cut-off tool to cut a thin groove in the inner race, being careful not to damage the hub; then, use a cold chisel and ball pein hammer to fracture and remove the race. There is not enough clearance between the hub and the race to use a bearing separator. Clean hub with abrasive cloth.

3. Remove wheel bearing circlip (Hi-Tech Tool 75 or KD Tool 3152). Cheap circlip pliers will not work. Clean with abrasive cloth and lightly coat with grease.

4. Wheel Bearing Removal. Press wheel bearing from knuckle, by using the next to the largest receiver cup from the FWD Front Wheel Bearing Adapter Set (Harbor Freight 66829). This requires a powerful impact wrench, if you use the receiver cup--I use an Ingersol-Rand 231-C, set on maximum power. Be sure to grease all adapter friction points with synthetic grease with moly; otherwise, the threads of the bolt tool will be stripped. I use Valvoline SynPower grease. Clean corrosion from wheel bearing seat and lightly coat with synthetic wheel bearing grease or anti-seize compound.

5. Wheel Bearing Replacement. Place wheel bearing in the freezer overnight, to slightly shrink the bearing. Press in the wheel bearing, using the appropriate adapters. If there's only one black seal, ensure it faces the hub.

6. Install circlip.

7. Hub Installation. Coat the hub's neck with synthetic grease. Place hub in the freezer, to ease installation. Press hub into wheel bearing, using the appropriate adapters or shop press. Ensure it's fully seated.