Price setting firms with cost reduction technology
Your reasoning for the 1st part is correct.

The 2nd part is a 2-period game. You should try to solve it by backward induction. First you go through all the 2nd period subgames. Then you can use the payoffs of the subgames and write up the investment decision subgame as a bimatrix game. However, the question is poorly written and as stated  this game has no Nash equilibria in pure continous strategies. Let's go through the reasoning as to why.

First, let's find the NE for all possible cases in the 2nd period.


Both firms have adopted the technology and have marginal cost 3. They will engage in a Bertrand game, set their prices to 3 and make 0 profit.
No firm has invested, both charge a price of 5 and make zero profit.
Now on to tricky case. What if one firm 1 has invested and firm 2 hasn't? It is crucial to know how firms split the market if they both charge the same price in this case. Since this is not given in the question, it is not really possible give a definite answer. I will go through two cases.


First, let's assume there's a 50:50 split with identical prices. Any equilibrium price above 5 can be ruled out by the usual Bertrand game argeuments. Firm 2 will never charge a price below 5, since it will make a loss. Firm 1 will never charge a price below 5, because given $p_2 \geq 5$ it could always increase it's profits by raising the price. So really the only candidate for an equilibrium is that both firms charge 5. However, if we assume a 50:50 split in this case, then firm 1 could increase it's profit by slightly decreasing the price and gaining the whole market. As a result, there is no equilibrium in this subgame, and hence no equilibrium for the overall game.

Second, let's assume arbitrarily that if both firms charge the same price, then all customers will buy from firm 1. In this case $p_1 = p_2 = 5$ is a Nash equilibrium.

You could then go on to find the NE of the whole game for different assumptions on the payoff split.