I have a lennox electric furnace w/ac. It is about 30yrs old and just stopped heating.
How hard is it to replace the heating elements? Can I do it myself?

What stopped working there is several heating coils in that furnace check to make sure you have 240 volts to unit the relay that runs the blower on that furnace has a way of coming loose you need to push it back in try that

I need to change heat sequencers , the problem is the unit is stuck in heat mode . Unit is about 30 years old and the seq. Are shaped different can I use the stack model seq. To replace them or what ?

You can, provided the replacement sequencer has enough switches. The original sequencer is probably more box-like (as in the pic in the upper-left). Ideally, you should have a separate switch for each heating element and the blower. The pic in the middle shows a sequencer capable of controlling 2 elements and the blower (the M1-M2 layer can be used for the blower and 1 element). If more than 2 elements are present, you will need a multiple-stack sequencer like the one on the lower-left. Pay attention to the switch symbols on the housing of the original, as they are a little unusual. The terminal designations on the stack-type are numbered in the order they close (M1-M2 makes first, breaks last, etc.). Make sure the blower is connected to the M1-M2 layer to assure it turns on first & off last.

Approximately how much would it cost to replace the old with a newer electric furnace? Assuming the house is 1,200 S.F...

My heater is around 30 years old. It is 11KW Nordyne. I need to know how to wire the switches and transformers to the elements'