RSA key length choice for TLS when confidentiality not important?
1.
To clarify: The critical time period here is one year (after wich the certs are changed). With the cracked RSA key the attacker can decrypt the traffic and do nan-in-the-middle attacks, posing as a valid hardware device.

Let us take the numbers determined by experts. In their paper on cracking the 768-bit RSA key the researchers state that they needed about $2^{67}$ instructions. When you look up current pricing for the Amazon EC2 cloud you get two Intel Xeon E5-2670 for 2.4\$ per hour. That is roughly $$2 CPUs \times 3\ GHz \times 8\ cores \times 8 FLOPS/cycle = 384\ GFLOPS$$ for 2.4\$/hour. That makes the cost of cracking a 768-bit RSA key about a quater million Dollars (on CPUs). Or 143 instances working for one month, or 1000 instances working for 4 days, etc. This is of course very simplified but gives you a rough idea.

Computing power doubles every 2 years and with special hardware, like GPU computing you can significantly lower the pricetag (modern cards operate in the order of TFLOPS see specs on AMD or NVIDIA websites).

A computer packed with 3 GeForce GTX 780 Ti (cost all together about 3000\$) can achieve 15 TFLOPS. This cracks a 768-bit key in 113 days, well under one year. So your key is definitly not safe from a moderatly determined attacker.

As for a 1024 bit key they state it would take 1000x more time or money. Very costly, but totaly out of reach in 5 years?

Other sources state a 2048 bit key would take $2^{32}$ times longer. So you are definitly save here.

But if I understand your scenario correctly you might be able to avoid the problem altogether: The computing cost of a larger RSA key only has to be paid once. During the inital handshake of the connection (generating the AES key). If you do not have a higly dynamic environment (meaning devices coming and going all the time) this cost will not matter much. 

2.
For generating a few certificates a year you do not need a special module. This is only neccessary if you need massive amounts of randomness/entropy (e.g. if you need to produce a cert every second). Here is command to find out the (estimated) entropy of your system on Linux:

cat /proc/sys/kernel/random/entropy_avail


I would be more worried about the entropy available on those hardware devices (needed for the TLS handshake). If they are very simple microcontroles that might be a problem.

3.
Kind of answered that in question one.

Hope that helps!