Need advice about coding style

Ok. I know how to use UNSIGNED ports. I make an example:

LIBRARY IEEE;
USE IEEE.std_logic_1164.ALL;
USE IEEE.numeric_std.ALL;
ENTITY full_adder_Nbit IS
GENERIC (
	N : POSITIVE := 4
);
PORT (
	ci : IN  STD_LOGIC := '0';
	a  : IN  UNSIGNED (N-1 DOWNTO 0) := X"0";
	b  : IN  UNSIGNED (N-1 DOWNTO 0) := X"0";
	s  : OUT UNSIGNED (N-1 DOWNTO 0) := X"0";
	co : OUT STD_LOGIC := '0'
);
END full_adder_Nbit;
ARCHITECTURE b OF full_adder_Nbit IS
BEGIN
	p0 : PROCESS(ci, a, b)
	VARIABLE
		result_v : UNSIGNED (0 TO N+1);
	BEGIN
		result_v := ('0' & a & '1') + ('0' & b & ci);
		co <= result_v(0);
		s  <= result_v(1 TO N);
	END PROCESS p0;
END b;

I have question1 : When i write initial value for UNSIGNED like ... := "0" without prepand symbol 'X' ModelSim unable to start simulation (Error loading design and point to line number for port a). Which symbols allowed at the beggining?

I think the code above can synthesis any combinatoral adder. I want some that work with clocking. question 2: why i reinvent adders, is there any class of ready function, where i can read about?

question 3: If i instatiate my entity I am not sure about the length of result_v variable which n will be substituted.

Thx in advance.